My 91 940 Turbo has developed a flakey speedometer. It works first thing in the morning for a couple of tenths of a mile and then usually just drops to 0 for the rest of the trip. A few times, the speedo jumped to 90mph and stayed there for the rest of the trip regardless of my actual speed. The odometer works when the speedo is working properly but registers nothing when the speedo is at 0 or planted at 90mph. I tested the sender at the differential with a known good one from another Volvo. This produced no change. Any suggestions? Thanks in advance.