Hi, I just purchased a 2012 Freightliner M2 106 dump truck with Cummins ISC 8.3L, 10 spd. It has 25k on the odometer, and I have been experiencing coolant temp fluctuation from 175-200 F. This does not appear normal.

Background:
After initial start/warm up and as I get to highway speed (55-60 mph), the coolant temp keeps rising up to 230 F and the clutch fan kicks-on w/ the check engine light (this happens frequently during initial start). I let the truck coast in neutral when this happens, and then the temp decreases to 175 F, rather quickly within ~10 secs).

Afterwards, the coolant temp steadily increases from 175 F to 200 F and back down to 175 F. I see this continuous cycle every few mins. Coolant temp is steady (~180 F) when idling after running for hours.

Here is the following fixes I have attempted so far:
1) I pressure washed the front condensor, radiator, and intercooler as best as I can. Minor debris/dirt found.
2) I replaced the temp sensor and thermostat (180 F) with factory parts

Any thoughts on symptom? Is this normal operations? What else could be the issue? I called both Freightliner and Cummins technical assistance for feedback but they were uncertain and recommended to bring into dealer for inspection.

P.S.: The coolant expansion tank is to the full mark and there is a slight coolant drip near the cap. Not sure what is the issue here (potentially air in the system?). What is the proper procedure to bleed the coolant system?