mouse studders at times on win98 pc

I have a dell machine 1.2ghz celeron, 256mb ram running win98 on a network. I have always had problems with the mouse moving very slow, the pointer will studder around on the screen at times and react very slow and screens will open up very slow aswell when this is going on. Then it will work fine and all the sudden everything will slow down again. Could this be a memory or processor problem. How can I narrow down the problem???