Absolutely mystifying high water temp

I have a 2000 Subaru Outback with 300k miles that I drive on a rural mail route for USPS. Had the water pump, timing belt, head gaskets, trans fluid & filter, plugs & wires all replaced about 35k miles ago. Car has drove great for years, until October of 2017. I started to have a strange heating up issue. The car doesn't actually overheat, but the temp gauge that normally points at the 9 o'clock position, randomly went up to the position between the red line and next dash down. I decided to put a new OEM Subaru thermostat, OEM radiator cap and drain the rad fluid and flushed the system a few times, added all new 50/50 Peak green fluid and the Subaru radiator conditioner and bled air out using a locking radiator funnel. The problem didn't go away. Still heating up, but it was only heating up when accelerating from a stopped position or going up a steep hill at slow speeds. The car does not heat up at all while at idle, it doesn't heat up while driving at a constant speed and if I turn my heater on full fan and full hot, it never heats up, even under acceleration or going up a hill. So, I thought maybe there was something wrong with the temp gauge or the transmission was overheating and causing the radiator to not work correctly. So I bought an infrared thermometer and checked the water return pipe temp, the transmission pan temp and the thermostat housing temp when the dash temp gauge said it was running hot. Water return pipe temp: 213. Thermostat housing temp: 186. Transmission pan temp: 236. Transmission temp seemed really high to me, so I put a plate and fin transmission cooler on it and now the transmission temp is running 170 at the highest. The problem still exists and none of the other temps changed after the transmission cooler installation. My next thought was a faulty HG, so I had the coolant system pressure tested and block "sniff" tested, both came back negative for HG issues. I checked for all the other HG symptoms. Milky oil, bubbles, white smoke, and fouled spark plugs. All negative for HG symptoms. I have asked every mechanic in my area and then some. No one has any idea what is causing the problem. It's been 7 months and the problem still remains. Does anyone have any idea what the hell it could be?