So I have a 2011 with the 5.0 V8 hooked to the 6r80 transmission. About 8 times when I come to a stop at a red light the transmission will downshift into first then immediately jump into 5th gear a stay there. I put it in park and it goes back into first or locks out in 2nd. This is the second transmission (Ford replaced the original) and it has about 135k miles on it.

I took it to a shop recently, and they replaced a seal causing an internal leak (a bulkhead seal, I think). I have not received the truck back yet, but I fear this problem is caused by something else. Has this happened to anyone else? I can't find anything relating to the transmission locking itself out.

Any help will be appreciated.