Thermostat Replaced -- Temp Fluctuating

Engine seemed to be running cool, so I replaced the thermostat. Now, the temp gauge will slowly move back and forth between about 190 and 150.

Is the new thermostat I put in bad? Opening and closing too slowly? Or is there another issue?

Prior to replacing thermostat, engine ran consistently at about 155-160.

1999 Suburban 5.7