Audi 80 Trans issue, please help...

[Mo Bass]

Ive got a 94 audi 80 and Ive had this issue with the trans. Basically I'd be driving along above 45 and the car will jump out of 4th and go into 3rd and 2nd, or it will simply go into limp (3rd gear). If i turn off the engine and re-start, the problem goes away but can re-occur at any time but tends to occur at high speeds and more so on inclines. I took it to a trans specialist who said it was a hydraulic issue with the trans, so we replaced it with a 2nd hand box. The day i picked it up from the shop it did it again. I then spent the next month chasing the workshop to tell me whats wrong with it but they didnt seem to know whats going on. They basically said it could be anything and to find out what it was basically id have to fork out the money to track it. After staying on their butts they said it could be the trans ecu, so i changed that, and still the problem persists. They then said it could be a wiring fault, so i took apart half the dash and tracked the wiring out of the trans box and up into the cabin and found 1 junction, so i made sure all the connections were tight (as best i could anyway). When the car is hooked up to a scantool, the error codes thrown are mainly 2, a speed sensor intermittent fault and a gear monitoring intermittent fault. Ive replaced the speed sensor (well 1 of them, i hear there are 2 in this trans...) and i find it hard to believe that 2 transmissions have EXACTLY the same fault, so im assuming its not the new trans box. If the error codes are cleared then the speed sensor fault keeps popping up again and only after a few times of going into limp mode will the gear monitoring fault come up, so i think its just picking up that the gear box is jumping gears when it shouldnt rather than it being an actual fault. Im reaaaalllly at a loss as to what to do and if ANYONE has any ideas or suggestions, please let me know. Any help would be greatly appreciated. Thanks in advance...