Just to jump on this thread as it sounds similar to my situation. I have just delivered an animation and one scene has been having so many issues, it's a simple bedroom/bathroom scene.
On random frames the 'rays actually' drops down to around 200 (normally sits at 12,000,000) and never finishes the pass, it's strange as it renders the first 7 passes. It locks max so I can't save out the frame and ram usage goes up to 100% (all other frames are using 30%). It also means backburner does not continue to render other frames.
Also at the start of each new pass only one core is used then ramps up to 100% as the pass continues rendering, then drops down to 1 core as the next pass starts.
Using 3dsmax 2021, corona 7. Happy to send the scene if you want.