Gaining time while jogging

[Bowlerhat]

I am a newbie Rolex owner and have just bought a GMT 2. I set it on Christmas day and it lost 2 seconds over the first 3 weeks. Delighted. I do a lot of exercise and always wear the watch. However, when I went jogging, after 3 weeks of ownership, it gained 35 seconds over a 30 minute run. It then returned to its near perfect accuracy, but around this new datum. I have ran with it twice since and it gained around 10 seconds on both occasions. I checked the threads and found some discussions based around the way repetitive rhythmic motion generates centripetal force and through it an acceleration of the the "tick" and deceleration of the "rock" on the escapement and therefore a gain in time. Although the time gain seemed excessive for such an expensive watch, the principle made sense so I relaxed a bit. However, yesterday I did my usual jogging route with my venerable Oris diver on my wrist and it was +/- 0 over 30 minutes. I am about to put the Rolex in its box and take it back to the dealer, but before I do am I missing something? The only doubt I have is that if the watch was 'broken' its steady state accuracy would not be so good (or it simply wouldn't work at all). Thanks team.