Vibration

[84Euro]

I have a 84 Carrera that I purchased recently that has developed a vibration. It has a 3.6 liter 964 motor that the PO installed. It still retains the original 915 gearbox. The problem occurs only around 2500 to 3000 RPM and is not speed related. Under acceleration in any gear the problem is not present. However while downshifting the engine through this rpm range in any gear it is quite evident. It only appears while the engine is not under load. This vibration can be felt throughout the entire car. Maybe it is me but seems to me that it is more noticeable as the car is driven longer, as the drivetrain gets up to operating temp? After some investigation some say that it may be the two part flywheel. I was told that the car had a lightweight aftermarket flywheel. How do I confirm this? Any other suggestions?