845PE very slow when accessing memory

Question

I have a P4 3.0GHZ (1.5GHZ) on a P845PE MAX board that runs very slowly (20 minutes+) on memory intensive operations, although it is only using half the available physical memory. If I run exactly the same operation on another machine running a P4 1.8GHZ it takes about a minute.
My machine
P845PE MAX
P4 3.0 GHZ running at 400 = 1.5GHZ
Win98(SE)
AMIBIOS 3.4 (default settings)
Radeon 7000 AGP
2 Western Digital Hard drives (7,200rpm)
512MB DDR PC2700
PCI Modem
300W PSU(will need to check as maybe the 12V and 5V rails are 140W)
Any suggestions

Answer 1

Installed a new PSU - No change still slow.
My estimate is that the CPU is fast when I do a Virus Scan it takes 18 minutes for about 10GB.
Now also tried a Celeron 2.4GHz, 128KB, Bus Speed 400MHz still slow on memory intensive operations.  Virus scan takes 32 minutes.
Tested the memory using a tool that's been mentioned on this site Passed 5 times.
In Bios AMI 3.4 CPU L1+L2 Cache set to WriteBack.
Can't upgrade the Bios fails to boot up if I flash with 3.9 or 3.4 always have to recover, very scary!
As I've said running on Win98(SE) - could this be a problem on MyComputer it states the processor is x86 Family 15 Model 2 Stepping 9.
Thanks in anticipation