OK, had the exact same problem at our house, the problem was bad wiring between the wall mounted switch inside the garage and the MVP opener mechanism. Romac finally found the problem after 3 trips to the house and lots of trouble shooting.
Have someone check the wiring for loose screw connections, and hidden splices, either one will cause the problem you describe.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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