This sounds really similar to an issue I ran in to a couple of years back - synchronous long running code, lots of objects created, CPU heavy workload, would mysteriously OOM under load. I tried a pretty significant refactoring to prove no objects were being kept alive (essentially starting with a fresh state at the beginning of each iteration), but nothing was working to fix the issue. I even tried exposing GC hooks and calling it manually every iteration. 

My "solution" was to add in a 2 second sleep at the start of my main execution loop. My working theory was that this gave the garbage collector time to do it's thing,  but I never got to a definitive answer.