hello i recently changed my thermostat in my I6 due to not having any heat . old one was clogged. but now it will heat up ok but the pressure will drop randomly and the gauge will rest on O in normal or go below the N randomly. idle it will stay on O but once i get on the freeway it drops
new thermostat is 195f gates. plus heat isn't very hot more warm but not hot. a buddy told me to check the head gaskets but i dont smell any sweetness in the exhaust and there is no white smoke

 

Head gasket which you only have one will cause overheating. If you installed a fail-safe thermostat that could be your problem. Also air in the system can cause your no heat and fluctuation on gauge . Also flushing system helps with the heat.

 

We flushed the system twice and I have to add a quart of coolant every week and a half

 

This is where it's been sitting all day and it's been in the lower 70s and stuck in traffic

 

That's about where mine sits. They are all different, it's just an approximation. The thing to watch for is sudden change.
The more serious problem is where is a quart of coolant going every 2 weeks ? It's not supposed to go anywhere.