'91 420SEL stalls at cruise, electrical issues

[golfdub]

At 55 mph '91 420SEL w/178,000 miles stutters, finally stalls. Tach goes above 3000 at idle, speedometer bounces from 0 to 25 or more with tran in park and foot off accelerator.

Radio had quit a week before. Power seats and windows quit after stall.

Old battery tested 10.5 volts. Used jumper cables and car started, all electrical worked. Installed new battery and car performed perfectly for about 100 miles and radio quit again. Tach again appeared to indicate higher than engine speed. Parked the car in park and let idle. Same fluctuation reappeared in speedometer.

Any help appreciated.