I have an 87 924S and it has the dreaded motor mount style vibration under 1200rpm but after driving awhile the vibration dramatically smoothes out or goes away entirely at stop lights etc. Additionally, when the car is cold it has a slight stumble when you give it a medium push on acceleration and then once it warms up it runs and accelerates fine. Any ideas?

 

You might want to check your distributor cap and make sure it hasn't become loose. I see this happen often. As for the motor mounts you are probably right that they need to be replaced.