My server is acting really... strange. Every day for couple of hours it takes a lot to load a page (there are usually 220-270 users online). It stays like that for couple of minutes and then everything seems to be ok. After 10-20 minutes it happens again. Server load is 0.5-1.5 (2 cpus) all the time but memory usage gets up to 90%. I'm not sure if that's the problem cause I've seen it working fine with 90% of memory used.
Server: AM P4 3.06 1GB RAM
Operating system: FreeBSD
When it gets slow again, check and see if it is using any swap space. The memory will be fully utilized and you'll see the swap numbers moving around. If the server is swapping, you will get the results like you explained. It will then be time to either optimize your server, or add more ram.