jet problems

Hi, I have a 2001 Johnson 25/20 jet on a 1448 Monark flat jon. The boat has a 15" transom and a riser bock in the center of the transom to raise up the mounting position for the jet. My main problem is that the jet jumps out of gear when I'm underway. I've adjusted the shift rod and cam to where the roller is firmly in the end of the slot in the drive position and I can't pull it out of gear by hand by pulling on the gate. I'm thinking the riser block is flexing and allowing the angle of the motor to somehow allow the force of the water to knock it out of gear. Anyone have anything similar happen to them ever? Any suggestions to try? I have to drive around with my elbow on the shift handle to keep it from popping out...

Something has to be out of adjustment because it should lock in forward by design.

This is the link to outboard jet's site that explains how to adjust.

https://www.outboardjets.com/pdfs/CAM%20BACKFIT%201222,%201223-2.pdf