I've just recently done an engine rebuild on my 280z and put in a lower milage 5 speed instead of the one i pulled during the rebuild. Once i started driving around, the tranny started popping out of 2nd gear-but only when i let off the gas while in 2nd or downshift to 2nd gear. I am using Redline MT-90 transmission fluid, but i'm not sure if that has anything to do with the problem. Help please!!!

Time for a visit to the tranny shop. I had a car popping out of gear once and it turned out that there was a 'bur' on the cog for the gear it was popping out of.

You should also check out this thread: http://www.classiczcars.com/forums/showthread.php?t=20158