Here is a link to a rather intense testing software (this is the 2006 version which is free)
http://www.download.com/3DMark06-Bas...html?tag=lst-2
3DMARK is an indepth tester that will push your card to the limits (and depending on your card, maybe over the limits) and give you a breakdown of what your card can and can't do.
If you would like to do a scan of your whole system they have a scanner for that as well (this version is a little older (2004) but it is free and a good tool.
http://www.download.com/PCMark04/300...html?tag=lst-1
Hope these tools can help you enough to pinpoint the issue your having
