over heating

I have a 1989 4.0 L. Cherokee. It continues to boil over through the expansion tank. I have replaced the expansion tank, radiator, thermostat, water pump, and clutch fan she still continous to blow out of the expansion tank but only when it is cut off while it is running it appears to be fine. I have a mechanic that has run a chemical test on it to tell if the head or gasket is bad and on 3 tries it shows negative. Does anyone have any ideas me and my mechanic are both stumped.

try removing the hose that connects to the exapnsion tank to the thermostat housing and flush it out really good, had that happen to me on my '88 MJ (Commanche pickup) for the life of me, I could figure out why it was overheating after I replaced the radiator. Flush that metal hose and you should be ok.