Problem with screen freezing 737 NG

After some years of flying the PMDG 737 NG series without any difficulties I am now experiencing a problem. Typically, after perhaps 20 mins of flying there is a click sound and the screen freezes. No blue screen, just the usual cockpit view remains on the screen and I cannot get out of it with control-alt -delete but have to reset my computer.I did install a new monitor three months ago and it is just possible that this is the problem. Currently FS$ is set at a screen resolution of 1650x1080x16 but I have run it at 800x600 and the problem is still eperienced. My graphics card is Radeon X800 GTO with 256 mb.I am not sure where to start to resolve the problem and I would be grateful for advice. I have to admit not to having tested whether this is an issue restricted to PDMG 737 NG, I don't have that much time to experiment by flying different planes for 20 mins and I am hoping that someone can help me quickly arrive at the actual faultBest wishes, John Morgan