Okay, my two cents, fwiw.
On my C3...waaaay back in the late 70's before synthetic gear lube the rear would start to make clunking noises as the fluid lost it's lubricity when taking slow corners. Further lube degradation would eventually yield a loud banging noise from the clutch pack. Have you checked the fluid level and or do you know when the last change was performed? You could feel the rear slewing the car to the left or right randomly, and this symptom would get worse as the fluid degraded also.
A second thought, but this is way out in left field, are the C-beam bolts at proper t/q spec?
Tom