On 1G cars, the speedo is mechanical, so I doubt you would ever see this problem. On a 2g however, the speedo signal is electrical.
I would get a datalogger or scan tool to borrow (or your mechanic) and hook it up with the key in the "on" position. Start the car, and if the speedo acts up, see if the ECU's speed signal is doing the same. If it is, its most likely the speed sensor on the tranny, but could be in the wiring i suppose. If its just the speedo its probably in the wiring somewhere. Thats a strange one, but I seem to remember explaining all this before, so you probably arent the only one that has seen this. weird..