PC momentarily freezes no error messages

Win98 SE pc, PIII, 450 mhz 256 mb ram continually freezes momentarily with no error messages, with unusually low resources (35% free). There’s tons of free disk space. What I’ve done so far:
Cleaned tmp files
Cleaned internet tmp files
virus scan
spybot scan
defrag
scandisk
Startup files are minimum required
Removed active desktop
Windows update is up to date

What am I missing?