1994 VW Jetta GL 2.0 5 SPD 154,300 Speedometer not working

I had the transmission replaced 4 month's ago by Import Specialties in Fort Worth Texas (never go there), last month my odometer went from reading 154,300 to 273,000. The speedometer still worked, but the car kept hesitating, stuttering and stalling. Finally the guages went nuts, the speedo quit working, the water temp went through the roof and all others shut off completely, then the car shut off and would not start. I replaced the battery as the old one was shot, it started and ran for 5 minutes, then died. I searched for loose connections and come to find out my ground which goes to the transmission case was loose and the nut that holds it was gone and the bolt holding the trans was loose, they were all loose. I tightened all trans bolts, found a new nut for the ground wire and the car starts and runs good, but the odometer still reads 273,000 and the speedometer still doesn't work. Do I need a new set of gauges or did something get fried in my computer when the ground was loose? Do I need a new computer? I took the negative off the battery, let it set and put back on, the speedometer worked for a day and quit again, I've tried the same trick repetidly, but no joy. Any help on this matter would be truely appreciated. Thanks.