With computers, the word(s) are" You just never know".
Sure, sometimes you do know, but most time you just never know. That is, until you use the process of elimination method along with testing and such.
So it pays to do the easy parts first, thats why i mentioned cleaning the gold contacts with pencil eraser, pink rubber ones are the best, anyway gently clean the contacts and blow out dust from the slot where the card goes.
Also, you might, if you have the mobo out of the box, turn it over and check for bad soldering, but thats only if you have it out of the box. You should use a magnifying glass to do that job as its usually hard to spot. Not all that common either, but you never know. Theres those words again.
There was a guy a few weeks ago here found a bad solder connection on an ide connector, which is no different than finding a bad solder connection on a vid card connector.
This would be the very last place to look for a problem, unless the computer has been taken in and out of the home\office, been to lan parties or things like that, moved around a lot.
Good advice + great people = tek-tips