Constant rebooting only at startup

I have a 3-year old 2GB Mesh PC with 512MB RAM which constantly restarts itself on the initial startup but seems to settle down when it's warmed up.
At first I thought this was a classic symptom of CPU fan not working properly and CPU overheating, but this only happens when the PC is cool not hot.
Have tried Memory test and all the RAM sees to be ok.
Could it be a power supply issue? What can I use to check it out?

 

My first guess would be memory. Memory tests stress for hot side, your experiencing cold startup problems. If running 2x 256MB chips, run one at a time (chips must be cool). If one 512MB, can you find someone to borrow for a test. If problem continues with different memory, try swapping out video card.
I had a motherboard (replaced under warranty) slowly deteriorate to a "narrow range" of operating temperature. My experience with PSU failure occur after warming up. If they failed at cool startup, they would not work at any temperature.

 

I was hoping not to have to tinker with memory in this way so may take it to the repair centre, but it is curious that it runs faultlessly after just a few minutes...no reboot occurs once it has warmed up..

 

Moving memory chips in and out is a very easy process. Most of us on the forum can give you step by step instructions. If you could provide us with model number for your Mesh computer, we could tell you where to look for memory chips. Any chance this computer is still under warranty?

 

Unfortunately it's just out of warranty.
I'd have to do these manoeuvres with the memory sticks when the PC is cold, as the symptoms seem to disappear once it's warmed up which is done pretty quickly. If I just leave it to boot up, it eventually gets to the opening log-on page after several reboots, so at the moments it's just an inconvenience...

 

Here's a new twist.
All works fine on the PC aftre warming up, that is until I load any CD into the CD drive, then we're back to the constant rebooting sequence until I remove the CD from the drive. Also, the Stand-By function that keeps me in my own area in XP no longer works. It goes for a full re-boot if I activate it after standing by.