Sounds like you got it pegged then.
Good way to check if the motor/trany is loose is to have a friend put it into 1st with the parking brake on and lug the motor via the clutch in and out. You can look at the motor and see if there is excessive movement
FWIW The 009 is not the best distributor for a stock or nearly stock 912 motor. It is a good unit for full race high RPM (7000+) motors, but those barely idle or make much power under 4500 RPM and have 300+ deg cams. When using a 009 on a 912 motor the advance curve starts too soon, ends too soon, and there is not enough total. You can adjust it to work but it will always be a compromise overall. If you set it up so you get enough final advance then it will be too advanced at low RPMs. If you set it up for correct low RPMs it will not have enough final and you will loose top end power. The 031 ($$$), 022 ($) 0r the inexpensive 050 are better choices. All can be recurved if you have the proper equipment and know how.
Surging and or flat spots are not uncommon in carbureted cars and have common causes.