Computer Slow Down Problem

My computer runs on Windows ME,has 320 Megs of memory and runs at 500 MZ. Every once in a while the the system slows to a crawl. For example I might clik on an item and it will take 20 seconds for it to respond or I can be sending an email and it will take 10 seconds between each letter that is pressed. If I shut the system down and bring it back up everything is fine. I have tried different memory and ram managers and nothing works. Any help will be greatly appreciated.