2018 Chevrolet Silverado Z71 4x4 V8 5.3 Temperature gauge needle flap during uphill driving and high speed straight away driving.

[janonas]

I just bought a 2018 Chevrolet Silverado Z71 4x4  5.3 V8 2 weeks ago and my issue just started happening  , everytime I'm driving uphill in a freeway stepping on the gas pedal to get more.power , the temperature  gauge moves from the middle which is the normal temperature to  the the right one notch which means its getting hot, as soon as I clear the hill and ease up in the gas , the needle gauge immediately goes back to normal in the middle , it also happens when I gun the truck on a flat road straight away .The problem seems to happen when the RPM goes to 5000. This shouldn't happen to a brand new truck with less than 1000 miles and I've had Chevy trucks before and I've never had the temperature gauge needle fluctuates like it's doing right now. I took it to the dealer and they replaced the thermostat and refilled the coolant the technician report says the problem is fixed. I Then picked it up and tested the truck and it's still doing the same thing ,I took it back and the dealership and the technician say that it's normal that if it gets worst just bring it back since it's under warranty ??!!  Well this problem is not normal ... temperature gauge needles do not move unless there's issues . Any suggestions out there I would really appreciate it. Thanks.