I just bought a 2016 Caravan 85K miles. I finally took it on a longer trip (2+ Hours) as we were driving up a steeper grade hill the needle indicated it was overheating. I pulled over and turned the heater on high. The needle did not sit on the red line very long and dropped back to 3/4 warm mark on the gauge. I let the dealership know about it and they replaced the thermostat. The radiator gauge needle has always pointed at 3/4 hot and fluctuates from just over half to just over the 3/4 mark. On a longer trip now (30+ mins) it will hit the red line, indicate its overheating and then back off pretty quick. This is a consistent occurrence now. I am not getting any steam or indications that its actually overheating. In fact we have tested the temp via laser and nothing indicated it was hot (tested at top hose) I have replaced a sensor by the front of the motor, replaced the thermostat with an OEM, replaced a water pump, and the radiator was flushed twice, replaced the cap with a new one, I also tried to burp" the system by buying a funnel and letting the air come out. In replacing the thermostat I came across some white hard chunks like sand, they were able to be broken down by squeezing them. I am at a loss here and would love some guidance. I should note the motor is a 3.6.