Laptop randomly failing to post

I've formatted, re-installed windows and upgraded RAM from 256Mb to 512Mb in a Fujitsu Siemens Amilo Pro V2055 laptop.

Installation and everything seemed to go smoothly. However it's now randomly failing POST.

About 80% of the time when I turn it on it seems to 'power up' but no monitor activity occurs. I have to hold power button to force shutdown and try again, then eventually it just loads like normal without a problem as if nothing was ever wrong.

I have been tickering inside as I was going to upgrade the processor and hard drive, but nothing seems loose. I've re-seated the RAM, re-seated the hard drive, and checked everywhere else I can think of, it all seems fine.

Only thing I can think of is it's some power-up time issue, but the only change is the upgrade in RAM, which hasn't produced any errors itself. I've been using it for a while now without any crashing/blue screens etc.

I've also tried setting BIOS to defaults to see if that did anything (problems still occur).

Like I say, when it works and POSTs, it continues without a problem. It does seem to be an issue around the initial POST, as if it gets there is smooth sailing.

I'm not convinced its something loose as I've had it boot, not touched it at all apart from the power button, lightly pressed it to shutdown, pressed again to turn on and had the blank screen occur without any movement of the laptop at all.

I'm really stumped with this, anyone got any ideas?