We b -
I don't know if I'm qualified to give advice but I had a similar problem with a 5-speed. It was poping out of 4th gear, mostly on deceleration. A linkage adjustment seemed to solve the problem.
Better check with one of the real mechanics on the board before you start grabbin' wrenches based on this reply but this was a pretty easy thing to fix.
I didn't have any clunking so you may have a bigger problem than I did.
