This may sould very basic, but check the fluid level. Do this with the engine runnig. They used to say in neutral. Not sure if that is procedure or not.
Once you assure you have the proper fluid level, cycle the
trans through all gear positions (engine running) keeping it in each for at least 30 seconds to fill all of the passages in the valve body.
If that does not do it, check the vacuum dashpot that is on the side of the
trans. Verify that the vacuum hose is in good condition. Replace if not. As a last resort, replace the dashpot.
Hope this helps