Hi, I have a Nivdia 8500 GT card in my pc. LATELY I have had some WAVY lines on my monitor, so I tapped the back of my Case and it clears up ( happens every so often). Whats up with that? How do I permanently remedy this problem? What is the problem?Buy a new video card.
The card may be coming slowly unseated. Do you have another slot to put it in?

Not sure if I have another slot. I have a Asus P5B motherboard. I have never opened my case yet because of the warranty. ( expires soon) so if it's just unseated can I just shove it back in? Any thing to worry about? I never open my a pc yet but its time to slowly start learning what to do. I'm very cautious Unplug PC.......Unscrew the screw holding the card in place, and gently wiggle the card by the ends to remove it, inspect pins, reseat it carefully until it is in position, replace hold down screw.....don't touch any components on the card itself....Follow JJ3000s' advice and see if there's another slot available..... Touch the metal case each time you go into the case to AVOID ESD(static)...When you're in the case be sure to do some housekeeping....blow out the dust if present......You don't want to be tapping the case too much, what with drives spinning, LOOSENING CPU heatsink, fan ,etc., whacking the case could compound your problems.

One last thought, before you do any of the above, check your PC and monitor CONNECTIONS.....the thump to the case may be forcing an improved connection at the back of the case...tighten every connection.Excellent! Thanks guys! Love the static advice, I was intimated to do this, static being a issue when reading others that have ruined things in this manner, never thought of touching the side of the case.