does it pop out at any speed and revs? or when just giving it some? what was it like in YUM?

Just been readin up about mt75s and t5's, some t5s suffer with 2nd gear popping out, thats from the fork/sliders in the gearbox being worn.

The simplest thing i can think of is if the gear stick is being caught on the rubber boot and popping it out of gear