T221 Popping out of gear

[Vero67]

My 67 T221 recently started popping out of 2 hi and 4 hi. I can see the lever move when I touch the gas in 1st or reverse (sm465). I opened up the top today the springs balls and detents all looked good. I was looking for adjustments for the linkage, but the only ones I found showed the levers at 1 1/2" to the center of the holes. Fully tuned in mine only go to 2". Any suggestions are appreciated. The truck pretty much sat for the last 6 months. Before that it didn't pop out of gear. Do you think a broken engine mount or something else might be causing it? Thanks in advance, John