CJ 5 Clutch issue??

While on a test drive with a potential buyer for my 78 CJ5,(350, T150 and Dana 20) all was good. We stopped at a signal and when releasing the clutch nothing happened. Tried several more times and nothing. About the tenth time I could hear a slight noise then bang, it jump into 1st. Pulled off the road to check for any signs of whatever. No sign of damage, put it in gear and took off as normal. Later on it happened again. I leaning to the clutch but I’m just not good at fixing or troubleshooting something that isn’t visibly broken. Any input greatly appreciated.

 

One problem that CJ owners deal with is how worn out motor mounts affect the clutch linkage. Worn out motor mounts will cause the engine to tilt to the rear and affect the linkage operation bad enough to break the outer clutch bellcrank pivot. The clutch bellcrank should be perpendicular with the frame for proper operation. But with a GM engine in there it might take some research to solve the issue.