I had the same thing happen from a blocked exhaust actually, at a sustained 60-65 im guessing that EGTs got too high and the motor locked up then freed up by the next layby. Had just swapped the exhaust from a very leaky but free flowing one where the bike was noisy but would sit at 80mph happily, swapped exhaust and the next ride top speed was down to 70 and I had the seisure at around 65 after a long climb