System locks up and reboots

A win98-2nd Pentium II no-name system will lock up and sometime reboot several times a day. Also the dialup internet connection will pop up 1/2 to a dozen times a day trying to connect to the internet, but nothing is running (other than Norton system works and Quickbooks reminder in the background) The system was scanned with the latest antivirus signatures, defraged, scandisked, regcleaned, cleaned temp. Norton Utilities tested the memory and harddrive and CPU with no errors) The power supply fan is working. The system was wiped and reinstalled once several months ago (different problem) and I really want to avoid that course. Any ideas?