I've seen a condition where, once the transmission gets warm and hot, it pops out of 3rd on the highway and drops to 2nd and just won't go back into 3rd. That repair was deeply internal, and required pulling the transmission.
edit: I was referring to an automatic transmission. I think the OP is about a manny. my bad.
Just double check there is no problem with the shifter or the adjustment. Check the fit of the shifter arms on to the shift forks - they all seem to get butchered there for some reason.
Check for worn or sloppy parts.
Can't hurt, if it only does it occasionally it may not be an internal problem.
If you put a new rubber boot on your shifter, that might be the cause of the popping out of 2nd. Some newer rubber boots are hard and cause thus problem