Seems like you've totally ignored the power supply, which is the weak link in many a system.
I'd try switching it out for the few seconds it will take to confirm/refute it as a contributor.
If the original PS is in there, it's probably due for a change. Those eMachines systems are not noted for having quality power supplies in some systems.
Unrelated, but you might also mention changing the CMOS battery.
Thanks very much for the reply Louis and thank you also for a different idea, I've asked so many other people who seem emphatuated with the idea that it's a graphics card issue despite the fact that this happens without the graphics card installed.
Anyway, you're right about the quality of eMachines - not great, but my brother purchased a new (although very cheap and unrecognisably branded) 500w PSU just a month or so ago, although I did warn him that weak-branded PSUs weren't to be trusted.
The problem now is that we have no spare PSU, his old one died and mine is currently in use!
However, I will try to sort something out regarding that.
Are you thinking maybe that the PSU is randomly 'flopping' and stops supplying enough power to provide the monitor with Brightness, contrast and gamma?
One more detail that I've just remembered, my brother currently has his computer plugged directly into his 28'' flatscreen tube TV - (it's not an LCD, just a regular tele) and the problem is not
occuring although being a regular tele it is unable to output the computer image in a readable state.