new holland 1030 pto lever kicking out

[redstallion83]

Its a used machine, I got a 5' tiller for it and upon a test run the pto lever will not stay engaged. It starts to turn the tiller at about 1/4 engagement and gets hard to get all of the way to the slot for full engagement. Once I get it there though, as soon as I drop the tiller, it pops out of the slot and into neutral. The only way I can till is by holding it in the fully engaged slot with my hand, but this isn't ideal.

Any ideas on why this thing is popping out of fully engaged?