Yes, an overheated GPU can also cause artifacts like that, did you try checking the GPU temp when these issues are happening using the NVIDIA system tools (or other software of your choice) as suggested?
Although, if you have been overheating your GPU enough to cause artifacts regularly you've probably severely shortened its useful lifetime anyway.
EDIT;
If it is indeed overheating then a stop gap solution until you can improve the cooling would be to underclock the GPU, as I suggested earlier in the topic. I am fairly certain the NVIDIA system tools will allow you to do that with your GPU. If not you should be able to do it with nTune.
EDIT 2;
750W power supply is overkill for a single processor single GPU system, I run my AMD Phenom II X2 and 9600GT on a 450W PSU and even that is probably more than it really needs. People tend to really over-estimate their power supply needs these days....
Based on a quick calculation on
http://extreme.outervision.com/psucalculatorlite.jsp assuming high end motherboard, 2 spinning hard drives, 2 dvd drives and a 3ghz intel core 2 quad CPU along with your 9800GTX you only really need 396w, so a 450W PSU would be more than sufficient and would probably generate less heat. You could have spent the extra you paid for 300 extra watts to get a high-efficiency 450W PSU.
Just FYI.
