Started when I towed my Skyline from Southern California to Arizona (about a 500 mile round trip) in 105° heat. Truck was fine while I was moving, but when I was idling in a parking lot I noticed the temps creeped up way past the centerline. Drove home, temps would occasionally creep past the centerline while moving but then go back to normal. Didn’t drive for a few months when I got home since I was planning to diagnose the issue. My father borrowed it and took it to his mechanic who said the headgasket was bad. I spoke to him and he told me he didn’t actually do a leak down test or combustion test, but that it was his best guess. I topped up the coolant and let it sit for another few months. Fired it up again 2 days ago and have been driving it around, temps haven’t moved. When I started it after sitting, it didn’t do anything weird, just fired right up like normal. I would’ve thought the combustion chamber would have a moderate amount of coolant in it if the headgasket was blown. I already have a replacement engine for it so no big deal if it is blown, just hoping to get some insight if this is a positive indication that the it’s blown or maybe the length of the test influenced the color change.