I'll offer it could be clutch issues, worn or contamination, motor mounts, or propeller shaft bushings. The service manual gives a great deal of information and specifications for testing these things out. Mostly opening the car up to see the clutch or the bushings is nearly all of the work needed to install a new one!! Motor mount tests has a 24mm deflection test that sounds like a dumb way to run your car through the back wall of the garage so read carefully and have a good buddy to help you.  I'm waiting for mine to slip noticeably then I'll take her apart and do it all at one time.