I saw the same issue when trying to run 8 Compac f with two R909s.
This wasn't because of CPU overload, since CPU usage was only at 30% (12th gen core i5 with windows 10 PC).
This computer can run 50+ compac f and upto 9 R909 without a problem.
The devices run fine individually but the problem only occurs when running them together in the same cgminer window.
I tried running two seperate cgminer instances on the same computer, one each for R909 and compac f and that seems to fix the unstable hashrate issues.
I used
--gekko-r909-detect
&
--gekko-compacf-detect
to enable only one type of device in each cgminer window.
I'm not sure why this works, but I'd suggest trying this out.