Outdoor Power Equipment and Small Engines - Transmission problem with John Deere 170 mower

I am working to resolve a problem with the transmison on this mower. The problem is that the unit surges and then will jump out of gear. If the shift lever is held in position the unit seems to work correctly. I have disassembled the transmission and have replaced what John Deere calls the key, two of them used. These are what rides on the shaft and picks up the gear that is selected. I also inspected the gears and the shoulders of the gears and washers used between the gears. All look to be in good shape. I have also readjusted the spring loaded indexing ball for the shift arm. I discovered during this adjustment that the problem go worse if I tighten up the tension on the ball. You can shift it into gear and it will work correctly for a short distance and then moves out of position. My thoughts are that the shift arm and fingers linkage is slightly bent out of position. This is the only thing that I can think of that could be causing the shifter to be out of position. Am I on the right track?
Model #170
SN M00170X011274
Tansmission mfg Peerless
Model # 9157 U268

I believe you are right. I've seen where the shift arm that moves the ring with the fingers gets bent a little to the side, and becomes out of sync with the ball detents. You might want to replace the shifter shaft (or whatever the manufacturer's ane for it is). The shaft that comes out of the case, that actually slides the shift ring back and forth and has the ball detents on it.

When looking at the parts breakdown from the John Deere Web site part #AM37898 the shaft, fingers and detents are all in one piece.
Thanks for your feedback as it's good to have someone elses opinion as this is the first time I have ever been into a transmission.

I think you're on the right track. I believe the piece gets bent when the tranny gets stuck in a gear (because of a internal jam or breakage), and someone tries to force it to shift.