05 f350 with 6.0.Stock 6.0 with no aftermarket items.Drives normal but when pulling a hill with a load of about (7K)half of what its rated for i hear the fan kick in,about 5 seconds the temp goes from normal to overheating,the led display says "check engine temp" and stays HOT for about 10 to 15 seconds then drops back to normal as fast as it went up and fan kicks off.No codes setting.I took to dealer AGAIN and they did was put a guage on the A/F tank and we drove it,stayed below 12 whole time till i pulled a hard hill and jumped to 15 but never went over.They are saying they "think" its the head gaskets but wont back that statement up and want $3800 to do it.Never builds high psi sitting or even revving up so i dont believe its the h/g.Been a tech for 25ish years and never seen this,please help

 

Could it be the fan clutch is going bad. If it kicks on too late it might cause you overheating but when it kicks on it cools it down to normal.

Just my 2cents. Wouldn't know how to check that though.

 

temp shows normal when fan kicks on,then just BAM,overheats then cools down just as fast

 

Problem ain't just the coolant sensor is it?

 

shows slight overflow at coolant tank

 

The coolant should be no higher than the minimum mark on the tank. The dash gauge isn't anything but an idiot light with a needle. All it shows it cold normal hot. by the time it gets to hot it's fubar. You need to find a way to have it monitored long term to see what's really going on. What are you refering to as the A/F tank??