I'm currently running Slackware 10 and installed the 2.6 kernel.
Everything works great except that my networkcard (8139too) stop receiving and sending
after a while, like 5 minutes.

Read that I have to set Force No Local ACPI but I don't know which file to do this in?

I think this is a bug that they are working on it kernel 2.6

Firstly you can disable IPv6 in your modprobe.conf
That will stop the browser waiting a long time to load a URL. Do a search on this forum for it.
Add these lines

#Turn off IPv6
alias net-pf-10 off
alias ipv6 off

modprobe.conf is located at /etc/modprobe.conf

If your network stalls after you have been away for a while then you can stop it and restart it.
as root
ifdown eth0
ifup eth0

ifup and ifdown are located in /sbin

Do a search on LQ. This is addressed many times on this forum.
Good luck