1999 Plymouth Grand Voyager Overheating

Part of my message was not posted. First time for me to use this site. When this first happened 5 yrs ago I have replaced the radiator, cap, thermostat, flushed system and tried block check. This happens only one month out of the year at it's hottest. The car has to be climbing a hill with the AC on and the weather needs to be over 90 degrees F. It's difficult to replicate the problem and when it happens the temp. Guage "shoots" up. I quickly turn off the AC and run the heater to bring the temp. Back down. I have used the car on almost a daily basis for 5 yrs with no problems. Run the AC too, just not on the hottest days. Never seen anything like it. I have heard bubbling from the overflow tank from time to time. We live up on a 3 or 4 mile hill. I add coolant on a regular basis. Maybe a pint per month at worst but, still adding.
My best guess is I have a small crack in one of the heads that only opens up under extream conditions.
Would rather diagnoise the problem before tearing the engine down. When I used the block test I could not replecate the conditions to the point where it would climb the gauge. The only other way I could get it to heat up was to run it very hard and it seemed a bit on the extream or excessive side. I would think over time if it was head gaskets it would degrade to a more observable condition. That's why I think it is a cyl. Head crack.

The rear bank does not look like the friendliest place on earth to be spinning wrenches, more like busting knuckles.

Paul