This one is mine (on the left
馃榿). I've worn it off and on since I bought it and it's never been babied. Serviced after 6 and 12 years (approx) by Omega in Eastleigh, England as I'm a Brit. I'm a watchmaker now so, no-one else touches it...
At 7:40, the hour hand is approaching the continuous running seconds counter on the left of the dial. If that hand is a little high, or more likely rising away from the pivot so it's not parallel to the dial, it will contact the hour hand and stop the watch. With no further details, I'd put a small bet that is the problem. Bet if you start it at 10:30, it will run until 7:40 again..
Cheers, Chris