my guy is doing this as well. we fell going over a 4ft fan oxer the winter of 2004, and to this day, he still locks up from time to time. I took him down to OHSU, and they blocked his back legs, but he wouldnt lock up after that. they had me ride him non stop for 3 hours. he wouldnt lock up! So they just ruled out arthritis. One thing I would suggest with the surgery - you have to remember the nerve that keeps their knees locked when they sleep, will be cut when cutting that tendon. So if he gets down to roll or anything, he wont be able to get back up - might want to take that into consideration. STRETCH him before and after each ride, and just take it slow. I notice in the colder months, thats when Gizmo's stifle acts up. just take it sloooooowww!!! good luck!!!