So I’ve been dealing with an odd problem with my mustang. It started a few months back when I took the car for a little spin around the neighborhood. It probably hadn’t been driven for a few months before this. While I was cruising around, I noticed the temp gauge getting up to around to the ¾ mark. I imminently went back home and parked it in the garage. The coolant level was good but the coolant looked a little old (yellowish not bright green). I changed out the fluid, thermostat (180*) and radiator cap (16 psi), all for good measures. I worked all of the air out of the system and took the car for a spin around the block. Started doing the same thing! I pulled back into the driveway, but this time I let the car idle to see if the temp would go back down and it did.I bought a block tester to see if there were any hydrocarbons in the coolant and it gave sort of inconclusive results. I then decided to just put a new set of head gaskets on the car, along with a new water pump. Well, I get the car all back together and I’m still having the same problem. The car has a newer dual core radiator and I am not running any underdrive pullies. I’m also not running any electric fans. I don’t know if maybe the temp gauge sending unit is bad? I’m just driving around the neighborhood at like 20 mph in 2nd or 3rd gear and the gauge will slowly creep up to about ¾ in about 5-6 minutes of driving. I burped the system with the car jacked up on the front drivers side with the heat on high and the cap off for 30 minutes or so. I suppose I could try to burp it again with one of those special funnels, but I’ve done it so many times in the past and have never had any problems. Anyway, if anyone has any ideas, I would really appreciate the input.