win 98se

I have a customer who has win 98se after restall of OS the computer runs fine for about a month. Tehn it starts messing up getting errors or just not starting. they have to reinstall and it runs fine but then in a month or two it starts all over again. I can’t seem to diagnose the problem cuz it is running fine right now. The configuration of the machine is unknown as for is the hardware. Any help here would be appreciated.