Robs,
I had a similar mysterious overheating problem with my 88 Cherokee 4.0 L. The car was still under warranty and I took it back to the car dealer who pressure tested the coolant system and found nothing wrong. I checked the coolant for oil, etc. but could find no indications of a leaking head gasket. Then one very hot day, I saw a very small puff of white smoke come out of the exhaust which only lasted momentarily. I took it back to the dealer that same hot day, and sure enough they finally found an intermittant head gasket leak. Apparently, when it got hot enough the coolant pressure would lift the head slightly to vaporize some coolant and then the head would reseal itself. The head was acting like a relief valve that would blow off some coolant and then reseal. This was really wierd. Anyway the solution was to remove the head, resurface it, replace it with new gaskets and the problem went away. Thankfully, the dealer had to make this repair. You may have the same problem.
When I do head jobs, I like to use the old hot rodders trick of using aluminum paint (stove pipe paint). I clean the head and block surfaces and then start spraying the surfaces with GumOut and wiping them clean with paper towels. I keep doing this until the paper towel stays clean. Then I know the surfaces are clean. I then coat the head, block, and gasket with about three coats of aluminum paint. After the paint dries, I assemble the head to the block and torque it down. I fire the engine and bring it up to operating temperature and then shut it down. Then I retorque the heads and they stayed torqued.
Hope this helps. Good luck!
Best regards,
CJR