'wavy' Lines Across Screen (like Refresh Rate, But Not)

I built up a PC a couple of months ago. It has an ok spec, Phenom II 550 x2, NVidia GTX 250, 4b ram, 500gig HD, 620w PSU. Windows 7 Eval. 7100.

From the first couple of days I noticed an issue that looked like incorrectly setup refresh rates - there are wavy lines accross the entire screen, and is particularly noticeable on highlighted text and the side edges of windows. I made sure the frame rate was correct, both in Windows and according to the GFX Card. I have tried all the settings but have settled at 75hz. This is the best setting but does not get rid of the problem.

I have the most up to date drivers for my GFX Card and Windows 7 is up to date too.

The intensity of the problem can fluctuate. Right now, its fairly minimal and only visable if you are looking. It can however get to the point where its becoming hard to read text, or at least tiring because you have to strain some more. This can happen at any time. The only link I have found is to games, as both World of Warcraft and Borderlands will intensify the isse, both ingame and in Windows. I am fairly sure they are set at the correct resolution and refresh rate where possible.

What could this be?

Further info, three weeks ago I turned the PC on in the morning to find the 'power' button unresponsive. After some waiting and several attempts it eventually turned on but I got a 'no signal' display on screen. Later I resorted to jump starting the PC without the power button, using a screwdriver to connect the prongs - after doing this, the power button has been fine although it took another day for the no signal message to go away and normal viewing return. This problem has not repeated since.

During this the fans were fluctuating a lot, audibly so. This dissapeared mostly along with the above problems, but I still think it sounds like the fans are fluctuating infrequently, particularly on initial bootup. I have no software for changing fan speeds, as far as they know they run at the speed I set physically.

A lot of this seems to point at the GFX Card, although I am a little more inclined to think it is the power supply, or the availability of power for each unit. But I really don't know. Either I can find the solution online or I take it to the shop and pay for a repair, not that its easy to even tell them what they need to repair.

The PC runs fast and with relatively few crashes besides that one weekend and this persistant display problem.