If you have the standard Muncie SM318 3 speed transmission the problem can be one or all of the following:
2-3 speed clutch sleeve and related rings and cones.(AKA syncro drum assy).
Worn shift fork, springs, balls, and side cover components.
Worn energizer springs on 2nd and 3rd gear.
Worn bearings.
Contributing factors are bad u-joints along with sloppy shift rods and levers.
Popping out of 3rd gear is a common problem with this series transmission.
Fred
