1990 CHEVROLET C2500 Power Train Problems

THIS IS A PROBLEM CENTERED ON THE ELECTRONIC SPEEDOMETER. APPROXIMATLY AT LEAST ONCE WITHIN EVERY 50 MILES THE SPEEDOMETER WILL FAIL. THE MOST DANGEROUS RESULT OF THIS FAILURE IS THE ANTI-LOCK BRAKE SYSTEM WHICH WILL LOCK UP THE REAR BRAKES IF THIS SITUATION (DISENGAGEMENT OF THE SPEEDOMETER) OCCURS AT THE SAME TIME AS BRAKING IS OCCURING!!!!!!! BRAKES FUNCTION NORMALY AFTER THE FAILURE HOWEVER. WITHOUT THE SPEEDOMETER FUNCTIONING THE DASHBOARD WARNINGS ALSO ENGAGE. I HAVE FOUND THAT A QUICK 'RAP' TO THE DASHBOARD WILL RE-ENGAGE THE SPEEDOMETER AND CURE THE PROBLEM. I BELIEVE THE SITUATION MAY BE CAUSED BY 'COLD SOLDER JOINTS' WHERE THE ORIGINAL SOLDER CONNECTIONS HAVE SEPARATED FROM THE POSTS ALLOWING INTERMITTANT CONNECTIONS. I DID ATTEMPT TO RE-SOLDER THE VISIBLE CONNECTIONS AT THE BACK OF THE DASHBOARD COMPONENT WITH MIXED RESULTS: THE PROBLEM APPEARS TO HAVE DIMINISHED FROM A LIKELY FAILURE OF WITHIN EVERY 10-20 MILES TO A LIKELY...