2000 Ford Windstar-speedometer/tachometer issue tripping rebuilt transmission

2000 Ford Windstar-just had a second transmission installed (first one-very shoddy work) but something keeps tripping this rebuilt tranny. Speedometer/tachometer goes haywire and then first gear only is straining. This happened before and after the tranny was rebuilt and the mechanic took it for a test drive only to have to pull the tranny again as the speedometer/tach issue damaged it. They seemed to correct it-test drove it and said it was fine. Gave it back to us and less than a day later-it happened again and we had it towed back to the shop where they are still figuring it out. Also the o/d light continuously blinks when this happens also. Any ideas? Thanks for your time.