Flickering display

I upgraded my graphics card from a 2600pro to a force3d 4850.

The problem is that with every game I play there a lines that flicker around the screen and the screen flashes between showing the game and a black screen. This normally goes away after a few minutes.
I have tried reinstalling the drivers using driver sweeper to clean out the old drivers and the card is not overclocked.

If I take a screenshot of the problem the screenshot displays the game just fine without the error.
For some reason Rival Tuner et. al. still say I have a 2600pro with a R770 core but device manager shows 4800 series.

Sys specs:

E8400 3.0
2Gb Ram
force3d 4850 512Mb
Proline 600W PSU
Seagate barracuda 160Gb
Gigabyte Ga-G31MX-S2

I am using a DVI\HD15 adapter.

Could you please advise me on how to fix this problem(Should I RMA the card? etc)

Thank you,
Billy

How old is your monitor, my brothers is getting old and it takes awhile to warm up.