At that sort of operating temps, I'm guessin you have a materials expansion problem. There is a difference in the expansion rates between metal and plastic. At room temps they are one size, and at lower temps they may not contract at the same rate, causing things to be just a bit out of tolerance.
To test the theory, put a small space heater directed to the plastic parts for an hour or so before firing up. If the problem is less, I'd suspect its a too cold problem.
Its the nature of mechanical systems to do this sort of devilish intermittency. But if two materials don't share the same expansion rate, and they need close tolerance to work, houston you have a problem.
Good luck with your Shop Sabre- I'm considering one myself, so I'm reading to see what kinds of problems, and how the company responds.