Hi, I had the same problem and the gears would jam up as well, and then I had a look at my manual in my GTHO Phase 111, and remembering some time ago that a guy had a Top loader jamming in gear in his XY GT.
The only thing missing from his gearbox and mine was a gearbox bracing rod, it's job is to stop the twist of the gearbox when your giving it a bit of stick.
I looked around for one to suit the XR8, but everybody that I spoke too said that I did not need one and it was a fault of the gearbox. Well, I did'nt listen to them and made my own, and since putting the brace in to stop the gearbox from flexing under high load and high RPM, all my problems have gone and have not resurfased. The price differance between upgrading to a Tremic, and the brace, well lets just say a new Tremic cost $3500, the brace cost me $28 in metal and an hour or so of my time.
I hope this may help in someway,