More info please.
Is it using coolant at all? do you ever have to top it off with water?
If you just replaced the thermostat did you use any permatex gasket sealer? This can ooze and block the small hole in the t-stat housing, trapping air in the system.
Did you carefully bleed the system? Multiple times?
have you checked the guage cluster ground? Check the condition of your hoses. There can be blockages in them, or in the water jacket of the block. Did you flush the system real good when you installed the other stuff?
If you are driving at 30 or so in a high gear like 5th with the heater on full blast does the temp go down any?
Did the guage work perectly and the car stay cooler before some work was done? It could be that the guage needle is saying 3/4 but is out of wack and really means 1/2.