Popping out of is likely the dogs on the side of the gear, it's the dog on the 4th drive or driven gear that will make second slip I think. My zilla is doing the same. What happens is the straight cut dogs(tabs on the gear side) and the groove they ride in on the next gear become rounded off after years of power and hard shifting. They best is to have the gears sent out to WCR, Jerry or Matt Shearer and have them back cut. It basically is a slight dove-tail on the dog and slot. Basically locking them when engaged and not being able to slide and round themselves off. The shifting is a little different, you cannot really powershift. Some of the others can elaborate more.