NVIDIA GeForce FX 5700 Problem

I have had this card for about 4 years and it drives a Sony LCD
monitor. Recently, when changing 'pages/views' the screen will go
black before regaining 'focus'. Even more recentl, after a time the
screen will dispay light red cross hatch lines.

I have run a 'stress' test on the card (proprietary software).
Initiallythe card quickly failed under 'stability'. I removed the
card and found the heat exchanger was very dirty which I cleaned. The
card now completes the stability test but the problem of the screen
blacking out between views and the red cross hatching persists.

I think the card is at fault. I don't have a replacement card to test
it against and am looking for advice on whether it is a card problem
and any advice that might cure the problem.