Thats more than likely the problem. Again even if it was a mount gone it still shouldnt happen, the linkage is bolted to the gearbox casing and moves everytime the box does for that specific reason. I'd say theres a very very very very very SLIM chance its the linkage but if it was it should do it in any gear. The car in my siggy had the slackest linkage ive ever seen and that never popped out of gear once.