Alright, just got word from Tony (the mastermind behind Magic Dosbox). He’s not available atm, but he can send you an older version for testing. Can you tell which was the last working one for you? Or around which time it was last working well?
There’s also the possiblity to implement fixed cpu affinity in a test build, but that’s experimental, and you’d be better off with a fix that can be applied to the official branch (for future updates).