Win 98 Shutdown problems

I have a Win 98 pc that will not shutdown if it has been used for any length of time. IF i was to turn on the computer check mail then shut down 9 times out of 10 it will shut down fine. IF the computer is used for a long time though IT will not shut down and hang on the “Windows is shutting down your computer screen.” This computer has been formatted and everything on it has been installed fresh. I have tried the options on the windows website and nothing seems to work. The problem is it is hard to trouble shoot something when it works fine if it is not used for a long period of time. But when it is used for more than 1/2 hour then it has the problem.

Simon