Monitoring Linux / Programs that reports program / warnings for linux.
 

I'm having some instability problems on my CentOS 5 64bit on an advanced DELL server (rack server) which has informix installed. SOmetimes but not ofthen, the server is working very slow... I checked the logs (system logs / informix logs) and there seems to be nothing wrong.

My question is: Is there a program that can monitor the server / reports more advanced details ? I mean, is there a program that can be installed on the CentOS server and reports me for example that on hour x on minute x the server had full speed lan or full cpu or full memory, or instability hdd, or something like that ?

When the server is working slow, the network seems to be ok, I copyed some big files from a server to another (samba) and the speed is ok.

One time, when the server was working slow, I tried to access it through ssh and the response was way too long, I was able to log in after one minute.

Well sure, any SNMP capable networking solution, like nagios or zenoss can do this. assuming it can still get SNMP responses when it's in a bad way.