I would say it is mechanical. I have a GP9 that does the same thing. 3-5 laps and the rear coupler uncouples. I looked at the mechanism that latches the coupler and it is not centered properly to stay latched. Mine is a TMCC engine but The couplers are probably the same design. I can unlatch mine by pulling and pushing on the coupler and it will unlatch. Replace the couplers and your problem should be solved.
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