What causes an automatic transmission slip only when going up from the 2nd gear to the 3rd and not for the other gear shifts?

might be simple, might be bad clutch pack(tranny overhaul), try some Trans-X in the fluid to unstick a sticky valve in the valve body(simple)...good luck also, check and adjust throttle modulation, also could be solenoid pack depending on the transmission/vehicle...read up on it to do it right...Everyone should have a manual for their vehicle, and the library should have a professional shop manual available, for FREE, in the reference section(make copies of the correct sections) read up and fix it like a PRO! :) good luck-Dave