One of the smarter folks on here (forget who... may have even been a Baker) once informed me that Fast Bounce actually is more solid than Real Time bounces because it uses exactly as much processing as it needs to perform the operation. Unlike real time bounce which HAS to use a set amount of power to perform the bounce at a specific speed.
Not your issue and I may have misinterpreted what that smart person was saying but thought I would mention it since CJay brought it up.
I concur that looking for lookahead or high latency effects would be a good start. Also if you have not installed the Everett hotfix that might help. The fluctuation is weird though and completely goes against the super precise detection/replacement that we've been told DR offers.
Maybe you should post a step by step with extra details for problem report testing.
Cheers.