slow eMac


I have an eMac 1GHZ, 640 MB RAM, OS 10.4.2. Sometimes the computer starts working in an erractic way, and becomes veeeeery sloooow. Does anyone know the reason for this bevaviour?
We also have a program called Championship Manager ( It works fine, but all of a sudden, out of nowhere, the program freezes and we can no longer launch it. In this case, sometimes we have to reinstall the program twice, to have it running again (and obviously we have to set up all the preferences again!)

Thanks a lot!