Does it do this in a certain gear, or all of them?
Guess I'm kind of confused, which doesn't take much at times... But, is it lockd up now, or does it release for some reason after a while, then does it again..?? Within so many feet, or amount of time..??
I've got a 240, and lord only knows how many hours are on it, but there are a bunch, and never had a problem like that.
Almost sounds like something is getting picked up in the oil or something, and lodging in between a couple of gears.
I don't see your location, but is it cold enough to possibly have ice in the transmission..??