NES (RGB modded) shows image for only a second.

Hi folks,

I'm having a problem with my NES. I had it modded here in the UK by an official reseller of the RGB kit. The console worked perfectly for a few months but I've taken it out today and am having a problem. No matter which game I try, the image will appear for a second and then disappear. The sound stays however. If I use the composite outputs I get a constant image, but the picture is grey. You can still see the outlines of stuff, but there is no detail and it's impossible to play.

Does anyone have any suggestions? I plan to clean the 72 pin connector, but this doesn't sound like the other grey screen issues others have because they usually get no picture at all but the grey.

Thank you.