Temp gauge problem
 

I am having a little problem with my truck, about a week ago i put a new thermostat in it, now after the truck comes up to operating temp, when i accelerate the temp gauge drops a little, then comes back up and settles. I've got the cooling system bled and it only does it a couple times then quits. So if anybody knows or has suggestions, let me know. P.S. This is on a 4.0