Old mobo locks up intermittently and/or reboots on shutdown
 

Please help!

I have an ASUS P5A-B mobo with the lastest BIOS installed (1105).
It has an AMD-K6-2/500AFX processor. All mobo jumpers are correct for this CPU.
It's maxed on memory (768Mb).
I put in an old WD 20Gb HD & installed Win98SE.

It intermittently locks up (must power cycle to clear).
When I shutdown, it either:
1) hangs at "Windows 98 is shutting down" screen; or
2) reboots.

Any help would be appreciated!