What you describe seems to be an electrical problem...I would suspect a failed ground return that ties computer, instrument panel, back-up lighting, fuel pump return...a common problem seen numerous times with various consequences. Identify some ground lead, possibly at C200 ground leads in left A pillar, forward of driver door, or whereever you find them, and rerun new grounds to engine block. There are ground splices under carpet that were proven corroded . This costs nothing, risks nothing, and might solve the problem....
http://www.avigex.ca/xport/C200location.jpg
http://www.avigex.ca/xport/C200pinout.jpg