Zetor 6341 - Shuttle shift?

[plagacy]

My good old Zetor keeps kicking in and out forward gear especially when I'm hauling a load. Originally the casting had cracked on the shuttle fork? and was lose where it sides on tranny. Fixed that and it's still kicking out. The detent stops are good. I usually just hold it in with my knee and it will still kick out at times. Just not sure where to start and it's going to be an outside job in winter so any advice would be appreciated.