Weird temp gauge problem.

[Clint72]

I think I got it figured out, lost some coolant when changing the sending unit and forgot to top it off, so I did that.
I think the thermostat may be going out though because I left the radiator cap off and started the truck and watched the temp gauge and the water level and once the gauge hit H the thermostat finally opened and I added more. Was probably low before also. But once it did open, it instantly dropped below half on the gauge, then slowly went up to h again, did that a few time she until the radiator no longer dropped water level. Ended up adding about a half gallon of water.
Once that was done it stayed steady about 3/4 way.
Possibly air lock?

I'll change the thermostat and flush the cooling system just because.
Being that I just bought the truck I'm slowly doing basic maintenance since it appears the PO negleglected it.

Truly appreciate y'alls help! Hopefully ill be able to be helpful and informative on subjects that I do know about.