I had something similar in symptoms in 2012
But much worse results (cumins 6tb 5.6L )
The plastic expansion tank cracked and leaked
It was over heating from coolant loss,
The temp would go up, then coolant system pressure would release at the cap, temp would drop, then immediately start climbing
I ended up broke down, was towed too Flagstaff, and spent thousands of $$ in engine repairs, plus motel expenses for a ten day stay
Be careful, check everything
I can explain it to you.
But I Can Not understand it for you !
....
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1997 F53 Bounder 36s