I have a pretty old laptop that still serves its purpose. It is a Dell Precision M90.
It is running Windows7.
Recently, without warning, the screen would suddenly show lines all over the place, like a TV out of station.
At that time, I can't see or do anything other than turn it off.

I checked for driver updates, and it is all ok.
If I move the monitor on its hinges, it does not change anything.

It can take 5 minutes to happen, or a couple of hours.
I didn't install anything on it recently (as in several months).

It could be the monitor? the graphic card? something else?
Before bringing it to the shop, I would like to know if there are ways to narrow down the problem (and maybe, if I am lucky, fix it too).

Thanks.