Transmission slip?

I have a 94 Skylark with a 2.3 l and 122,000 miles on it. Transmission seems to be a 4 speed automatic and recently started having problems shifting form 2nd to 3rd gear. Instead of the nice clean and crisp shift it starts to shift the the engine speed increases a bit and it struggles but completes the shift after about 3 or 4 seconds. I changed the filter and added put in an addative that was supposed to help with slipping but no noticable differnce. I drive pretty light footed on the gas and I notice that if I push the accelerator pretty hard while accelerating it shifts much better and smoother. I don't want to drive like that becuase it is hard on the engine and probablly wastes gas to consistently do jack rabit starts. Once it has shifted through the gears and I am at highway speeds there are no problems, does not seem to slip. Is there any adjustments I can do to adress this or is this just the beginning of the end for this transmission?