There is a program named munin that monitors various things such as cpu usage,
network usage, memory, etc. You might want to try installing that to see what
is happening on the server.
Another option is to run top which will show cpu and memory usage. It won't
show a pattern over time, but it will let you see immediately if the cpu usage
is high, if it's spending a lot of time in the wait state, or if it is swapping.