Computer Quits Responding.

I have a client with a computer that is Intel Pentium II/400 Mhz, 128 MB RAM, Windows 98SE, Internet Explorer 6.0, all drivers and updates. Computer has an EPOX EP-6VBA motherboard, 20 MB Western Digital HDD. Problem – when on the internet and surfing(via cable connection), computer will quit responding. It takes a hard reboot to get system working again. I have ruled out heat problems, have installed new CPU and case fan. Replaced RAM, motherboard is rather new, and HDD is new. Anyone with any ideas….HELP….pulling my hair out!(and I do not have that much!)