While my advice is well meaning, you have to assume all risks.
It sounds to me there is something loose. Are you able to pull the machine out, remove the case cover to gently push all the cards "home"?
This may be a good time to pull out the video card and closely inspect the nuts and bolts that the cables screw into. Sometimes I find them backed out. I'll unscrew them to put a drop of glue on the threads and then screw them back in. The glue will help keep them from backing out. Here I rarely use more than one drop of glue.
Now would be a good time to clean the machine. Watch how at http://reviews.cnet.com/4520-11319_7-6240575-1.html?tag=rb_mtx;wp_body
Bob
I have Dimension 4550 and a about a week ago I thought it dropped dead. One minute it was working perfectly, walked out of the room, came back and everything was off. Did the whole routine of shutting everything down and back up. This went on for a few hours then decided to unhook everything and switch out monitors. No help. I was just about to give it...as it's older than I'd like. I pushed the computer back to where it should be and it moved the cord that comes from the monitor to the computer and everything came on. I left it alone for the next day or two then powered it down and hoped for the best. Booted up and all was well until my grandson arrived with instructions not to go near it. He didn't listen moved the cord and down it went again. So I shut everything off and was checking connections again and from what I can gather where the monitor is connected to the computer it seems to be lose. The screws tighten as they should but there's movement in the connector...hope I'm explaining that right. And any time movement shuts down the entire system.
Is there anyway that can be tightened without taking it in for repairs? I've done some work on this pc with the help of people on here. Thanks in advance.

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