I just replaced my thermostat in my 1990 chevy silverado because every time I started it the temp gauge would go to 210 degrees then back to normal. Now my temp gauge gets to 170 and drops back to 150 then back to 170. So on so forth. I also notice bubbles coming up from the hose in the reservoir now after changing my thermostat.(no bubbles came up with the previous thermostat). I can only assume air bubbles in the radiator.

Yes, you need to purge the system of air. Turn the heater on to maximum heat and squeeze the top radiator hose a number of times, while the engine is running and at normal operating temperature. I would only install an OEM thermostat--others are problematic. Your thermostat is running too cold, which will affect performance and gas mileage. Check the spec for your engine. Did you drain the system and refill with a 50/50 mixture of the recommended antifreeze and distilled water?