I recently pulled the engine on my 2004 outback and replaced the head gaskets along with other items including the clutch, pressure plate, throw out bearing, and idle bearing. All new after market parts. Once the engine is warmed up when letting out on the clutch it has a vigorous shake. The one thing I did not do that I wish I had done is to have the flywheel machined. Could it be the flywheel causing this? If so why would it not do it when the engine is cold. When the engine is cold the clutch is very smooth.