I have a 1991 Dodge W250 4wd diesel that has intermittent dash
electrical problems.  Speedometer, temperature, and oil pressure jump
around and anti-lock brake and brake warning light flash on and off.  I
have tried replacing the speed sensor in the differential and voltage
regulator but neither helped. I also tried to pull a code from the
antilock control module but got nothing. Interestingly, if the truck
sits for several days the symptoms will disappear for about 30 minutes
before starting up again, but this is only if it sits for 4 or 5 days.
What would cause these symptoms to disappear after sitting for awhile?