Kz750 slipping into neutral.

It went like this, kinda like a pop or a misfire two or three times then it pops into neutral.

All was well with the bike. I replaced my starter which requires you to remove the shifter and did a carb sync which allowed for a lower rpm and did some final drive work. It still has a small leak I think.

Any ideas as to why it’s popping into neutral?

Often that is a sign that the engagement dogs on that gear are rounded off. Could be a bent shift fork. Has the bike been subjected to a lot of ham-fisted (footed?) shifting or a lot of drag racing?

No not really. I just took it on a ride and reall made sure the gear was fully engaged. Seemed to work fine.