If your machine is anything like my MTD Series 900 with the continuously variable drive then you might find that the problem wear in the bevel gear assembly. See picture. I have similar problems but more significantly my machines started to bang loudly "down the back" when turning a corner or going up a slope. Clearly some gears were jumping a tooth or two. I couldn't find any problems with my bevel assembly but ultimately found that the transaxle differential was so badly worn that it was jumping internally. I replaced the transaxle and bingo no problems with lurching and banging and also no problems with gear selection. BTW I used John Deere corn header grease when I reassembled the unit. Seems to be working well. I packed the forward and reverse crown gears with extra shims to reduce the clearance toa minimum so the clutch collar engaged then with to the maximum without causing any clashing. I have more picures it these might help