Had the same thing happen to me on my original MTX75 at about 20k miles. It was hard (near 6500) shift from second to third. I heard a pop and then there was no joy. I was on the highway and had to coast to the side of the road (at about 2am). I could shift in to all gears w/o pushing in the clutch. It had to be towed. It was still under warranty so I never got an explanation as to what the root problem was but they replace the transmission.
Good luck...
