C15 Acert coolant issue

I have a 2007 C15 Acert. Bought it at auction in May of 2016. Ever since day one it has been pushing coolant out of the overflow line and temp would rise at the same time. Have put on 30,000+ miles hauling hot mix at 84,0000lbs. Rencently it has gotten more frequent. Coolant temp will rise to 225. Then within a minute or to with out letting off the petal it will drop to 180 within a matter off minute or two. Normal temp is around 190z Have removed radiator and cleaned, replaced coolant diverter valve. Has passed the bottle test for a head gasket. And would think that if it was a head gasket it would have failed by now after 10 months of driving. Anyone else come across this issue before?

 

What's the best way to check the compressor? As much as I don't want it to be the head gasket, it probably is. But want to check everything else prior to opening up the motor.

 

It mainly only does under a load. And doesn't do it every time. Mind you I do short hauls. 50 miles or so.