Today all was well on a drive in my 1/2 ton with 3 speed shifter on the floor. Stopped at a stop sign after about a 1/2 hour driving and proceeded to go forward from first to 2nd with no problems but then it would violently pop out of 3rd.
Before I pull the tranny out and open it up i want to see if there is any other reason it would it would start giving a problem of this description.
I have checked the shifting rods and everything is good and tight and nothing has moved. With the engine off it will shift into all gears smoothly but with the engine running it will pop out of 3rd.....the clutch acts the same as it always has.
Anyone have any suggestions before I pull the tranny out ????

1953 chevy 1/2 ton with 3 speed 318 on floor and 1955 235 engine