Having some trouble with my heater and temp sensor.

92' Ranger, 2.3L

I just fixed all my coolant leaks and replaced the thermostat with a higher temp 180 degree one. The issue i'm having has been an issue since I bought the truck. The heater will work for a short time and then get lukewarm after a few minutes.

As for the temp gauge, it will get to the low mark on "normal" then it will dip back down to the C, as I assume as the thermostat opens. (see photo)

ALSO, I would like to add, the engine is getting to operating temp. After a 10 mile drive I took temps and the engine was averaging about 180 between both sides, the head and the block.

For the heater, am I having problems with my heater core potentially being clogged?

And is my temp sensor bad and not sending enough power signal to read accurately or something else?

Thanks.