Speedometer slash Dash Clock

[music24]

Hey another question for you guys!! I have a stock 91 S13 Hatch. My speedometer sometiems doesnt work until (what seems to be) my transmission is warmed. When its not working its just flopped down all the way past the 110MPH mark. It kicks in to work when the transmission is still cold when im trying to go up a hill from a stop and the clutch is engaged and the car rolls back a bit and goes forward, rolls back a bit and goes forward. That creates a real studder with the needle...it just bounces around from past 110 to 55 back down back up. Then when im on flat roads and the transmission is warmed up it seems to work fine untill im going at a faster speed like 65MPH. It seems to either be accurate or jump ahead some MPH. Could this be the speed sensor? Also the clock on Tachometer doesnt work...all the numbers are faded and are missing pieces until the car or the transmission is warmed....warmed up as in alot of driving. I dont know if those two are related but still would like to have some input. And if it is the speed sensor, do i have to drain out the transmission fluid to replace it? Thanks alot