i have a 1987 porsche 944 8v, and i hear a bang everytime i shift and the shifter vibrates alot when i am in about the 2500 - 3500 rpm range driving normal. once i get on the car and drive it hard the vibrations stop. what can this be?

 

The vibration is probably the balance shafts being out of time from the last belt job. Motor mounts will also cause a vibration, but usually are worse at idle, and smooth out around 2k rpms.

As far as the banging goes, I would look at the transaxle mount in the rear.