'01 DMax, LB7 w/400,600 miles
Head gaskets replaced ~62K miles earlier. Water pump replaced ~200K miles.
Coolant was a bit low, so about a 1/2 pint (give/take) was added. Engine was left to run ~1/2 hour in 0 degree temps. Within that 1/2 hour, the coolant developed too much pressure and was pouring out of the coolant bottle. Added about 1.5 gallons, ran the vehicle and had to add a little more several times. I'm currently consuming a bit of coolant with every 30 mile (+/-) trip on the highway. I've watched the coolant temp gauge in the cab go from ~180 degrees to 160 and back in the course of a couple of miles (air temperature from the heater dropped and rose accordingly). I see nothing dripping on the ground as of late; yet, level seems to drop with mileage.

Took this in to the dealership- they report nothing found. State it could be an air pocket that may be developing in the head (suggesting a questionable head gasket???). Topped off the coolant and ran a 50+ mile round trip and watched the temp gauge vary from about 170 to 210 and back within a 5 minute window of time. Looking for tips- thermostats going bad (they haven't been changed in quite some time)? Water pump? Head gasket failure after only 62K miles? The dip stick shows no foaming and the oil appears golden (not "orange"- not to say there isn't any getting there). No smoke from the tail pipe (except the normal smoke you'd expect under acceleration at 0 degrees air temp and lower).

Any input would help.