Now we are getting somewhere. The valve click is likely the missing clue. The valve guides on those MitsuShitty engines have a reputation (well earned) for failing. Early ones simply fell out - newer ones wear out. I suspect you have a loose valve guide on the exhaust valve of #5. Pull the rocker cover and turn the engine till the valve is fully closed, then attempt to wiggle the top of the valve. ANY noticeable movement is excessive. Have the heads rebuilt and you should be good for close to another 50,000 miles or more. The bottom end on those engines is pretty solid - I have over 240,000km on mine - on the third set of heads. Guides fell out of the first set, and wore out on the second.
This click has been with us for a long time.