I must disagree. His network connection may still be valid, but something is going flaky in name resolution. It is possible that forwarding the nameserver port is unstable, that DHCP lease negotiation disrupts it even though this box is now using a static address, or that something on the host itself is renewing the nameserver configuration from time to time. I have seen examples of all three at different times in the past.

I have seen Network Manager cause this kind of problem, but not in recent versions. Any recent version would have to be misconfigured to generate that symptom.

thanks, what else can I do?

First, I would isolate the general cause. For this you will boot from a live-cd image (knoppix, tinycore, puppy linux, ... almost any will do: use one you like) and see if browsing when running that for an hour or two still shows the symptoms.

If you still have the symptoms, it is either hardware related or an interaction problem specific to Linux drivers and your network device. If the symptoms vanish, then it is setting or software related and specific to your install.

If it is hardware related, you have to decide how much you want to invest in finding and avoiding or replacing the hardware causing the problem.

If it is software or settings, I would go over the network settings at SEVERAL levels to see if I could spot the issue. Disable the DHCP client if you are using static network settings. On mine I have removed Network Manager and installed an alternate network client settings tool, but that is somewhat extreme for someone who is not an experienced SYSADM.

Once you perform the live-cd session test, let us know your results and we may have more specific "next steps" to recommend.

okay, I'll do these tests, thank you so much for the moment, I'll let you know

I tried with a live distribution of knoppix, but ninetees it seems like it has the same problems. I tried in the file /etc/NetworkManager/NetworkManager.conf to comment on the line related to dns = dnsmasq and nothing.
I'm trying on Windows with virtualbox and Mint and it's all right on the first shot without making any configuration.
For security, I bought a dedicated network card to see how it goes. Never had these problems, I do not really understand where the problem is.

Re: the network manager configuration -- if you are running DNSMASQ then that setting is probably just fine. The resolv.conf entry directing to 127.0.1.1 is likley wrong. It should be 127.0.0.1 (the loopback interface) with DNSMASQ set to query the real external nameservers.
See if you can check the DNSMASQ configuration and if it is correct.

If knoppix is having the same issue, then it seems either hardware related or something it is pulling off of the DHCP server during startup. Can you check your DHCP server for the settings it is pushing to the DHCP clients? (Possibly something tied to the mac address of this one machine.)

By this test you have eliminated those things specific to your hard disk installation alone. Good job so far.

I solved everything like this:

Interesting! And what made you try that?
You may want to come back and mark the thread "solved", to give hope to anyone else that comes looking for clues to the same issue.

Done
I was using windows and I was examining all the details of the intel driver and I noticed this setting that is set in windows on 100 mbps full duplex. It was the only thing I had not done yet and I did it, I would never have come by logic, I did not understand why it should be done and it does not automatically negotiate.

I have seen the duplex setting auto-negotiate WRONG, but the last time was in 1998 I believe. It was never common, and I thought it had been fixed in all hardware LONG ago. Congratulations again!

Hi.

I have the same problem, wired ethernet goes out randomly. Disconnecting then reconnecting in the Ubuntu toolbar usually makes it come back. WiFi works fine, dual-booting into OSX Yosemite works flawlessly either eithernet wired or WiFi.

I tried the "solved it" command: sudo ethtool -s enp5s0 speed 100 duplex full
but I get the error

Cannot get current device settings: No such device
not setting speed
not setting duplex

My settings:

$ cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

and

$ sudo lshw -C network
*-network
description: Ethernet interface
product: NetXtreme BCM57765 Gigabit Ethernet PCIe
vendor: Broadcom Inc. and subsidiaries
physical id: 0
bus info: pci@0000:02:00.0
logical name: enp2s0f0
version: 10
serial: 3c:07:54:56:73:3b
size: 1Gbit/s
capacity: 1Gbit/s
width: 64 bits
clock: 33MHz
capabilities: pm msi msix pciexpress bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt 1000bt-fd autonegotiation
configuration: autonegotiation=on broadcast=yes driver=tg3 driverversion=3.137 duplex=full firmware=57765-v1.42 latency=0 link=yes multicast=yes port=twisted pair speed=1Gbit/s
resources: irq:16 memory:a0400000-a040ffff memory:a0410000-a041ffff
*-network
description: Wireless interface
product: BCM4331 802.11a/b/g/n
vendor: Broadcom Inc. and subsidiaries
physical id: 0
bus info: pci@0000:03:00.0
logical name: wlp3s0
version: 02
serial: 68:a8:6d:57:f4:a5
width: 64 bits
clock: 33MHz
capabilities: pm msi pciexpress bus_master cap_list ethernet physical wireless
configuration: broadcast=yes driver=wl0 driverversion=6.30.223.271 (r587334) ip=192.168.1.5 latency=0 multicast=yes wireless=IEEE 802.11
resources: irq:17 memory:a0600000-a0603fff

Any thoughts?

Just one. You ran a command to set parameters for interface enp5s0 , but your REAL interface seems to be enp2s0f0. I would have a closer look at what interface is recognised and working when everything is fine and record that. That is the one you want to set and activate when things go south.