code p0455 is a (evaporative emission control system leak detected gross leak)the system has tubes that run from the back of the car to the front,if the car was on a lift one of the tubes could have been damaged.or they didint hook some stuff up,or there could have been some damage when they changed the headgaskets.
did you get this code after they worked on it?
im asumeing you have a v6 car, on the v6 the system empties into the pcv valve if there is a leak in the system you will get extra unmeterd air coming in to the engine that can cause driveability problems and sometimes starting problems.
if you look at the pcv valve you will see a acordian like plastic hose conected to it, try and plug both ends of the hose and see if it drives better,
as far as your trans gos this problem is probably just coincidence.
you should post any new info you get, hope this helps
