Auto transmission slipping and stiff gas pedal

Im having problems with my transmission slipping. Its a 97 1.6L 3speed automatic.
The radiator was corroded and i think it mixed with the transmission cooling lines because the transmission fluid level was really over filled and smelled burnt (probably from coolant boiling).
I just replaced the radiator and flushed the coolant myself and took it to a shop to change the tranny fluid, but the transmission is still slipping. The shop said the tranny fluid came out really clean (no metal bits or anything).

And to be more detailed the transmission isnt really slipping out of gear or randomly changing gears. Its just really slow when changing gears. When it goes from first to second it takes about 2-3 seconds and as its slowly changing the engine revs up too much and sometimes makes a banging noise when it finally goes into gear.
I think it might have something to do with the throttle body or the accelerator cable. Its hard to explain, its not stiff and not sticking but a combination of both. As your pressing it gets stiffer then as u push harder it frees up and is easier to push.
Not sure whats causing this?

Anyone have similar issues?

Nevermind. Seems to be getting better now that its warmer out. Its changing gears better but the gas pedal is still kinda stiff hard to press. And the gear selector is also kinda stiff