Boot Up Fails Early In Post

When the comp is first turned on, everything powers up as normal then it stops at the first boot screen prior to the memory test and powers off the monitor. The computer stays on but does nothing.

The system specs are as follows:
Operating System Microsoft Windows XP Professional Service Pack 1
Internet Explorer 6.0.2800.1106 (IE 6.0 SP1)
DirectX 4.09.00.0902 (DirectX 9.0b)

CPU Type AMD Athlon XP, 2000 MHz (15 x 133) 2400+
Motherboard Gigabyte GA-7VT600(-L)
Motherboard Chipset VIA VT8377 Apollo KT600
System Memory 512 MB (PC3200 DDR SDRAM)
BIOS Type Award Modular (07/03/03)
Video Adapter NVIDIA GeForce4 MX 440 with AGP8X (64 MB)
Monitor Benq FP547 [15" LCD] (1336662)

The problem is intermittent and sometimes the computer will randomly shut down.

I initially thought this was video card problem so I have swapped out the video card for a known good one with no success.
Next thing I thought was overheating, I checked the system temps in the bios after a good boot and noticed the CPU temp was on 90 degrees Celsius, I thought this was extremely hot so I cleaned the heat sink and fan and the temp has dropped to the figures below:

Motherboard 38 �C (100 �F)
CPU 73 �C (163 �F)
Aux 45 �C (113 �F)
Seagate ST380011A 43 �C (109 �F)

The CPU temp still seems hot, however the system will run for days without a problem then out of the blue fail to start.

I then suspected a power supply problem and still suspect that this may be the culprit.
Once the computer has failed to boot, it will not boot fully again until the PSU power lead has been removed briefly.
If I unplug the power cable momentarily the system boots fine on the next boot, which seems to indicate something is being reset when the power is shut off totally.

Voltage Values:
350w PSU
CPU Core 1.68 V
+3.3 V 3.25 V
+5 V 4.73 V
+12 V 12.80 V
+5 V Standby 4.89 V
DIMM VTT 1.33 V

I have also run sea-tools which has detected errors on the hard drive, but I believe the problem is occurring before the system checks the hard drive.
I am receiving no bios beep error codes.

I am leaning toward PSU replacement,