I had an 1460 that did that, We put a tarp strap on the lever, it worked for a couple years. But ended up replacing the gear or gears don't remember much else.
1660 did the same thing, teeth were getting beveled on them because they were not adjusted right from the factory, had to put new gears in it. I did the work my self, not too bad. Worst part is removing the trasmission without hurting anyone. Think it cost around $1200 for both gears.
Ok, so it likely is a gear wear and replcement issue. Thanks for the info, I'll be draining the oil in the trans soon, so then would be a good time to look into this.