Coolant temperature readings on the computer are still subject to any inaccuracies caused by a defective or tired temperature sensor in the engine.

You weren't clear about whether the head gasket was replaced before or after the new radiator was fitted? If it was after then the new radiator could need a flush as sediment from inside the engine could have been displaced and found a new home in the radiator.

I am wondering whether the overheating started since the head gasket job? If the mechanic was careless there could be gasket scrapings and other rubbish partially blocking internal waterways, the valve timing might not be correct reducing engine efficiency which often leads to increased temperatures and the head gasket joint or a crack could be allowing more gas than normal to escape which can displace coolant and disrupt thermostat operation.

It is fairly normal for even a healthy engine to produce a few bubbles of combustion gas in the cooling system and designers usually provide a facility for these to rise quickly to the surface of the coolant so they do no harm. The vent for this purpose is often very small and easily blocked by corrosion or other detritus. Overheating can be the result if the vent becomes blocked.

Try this procedure:

• The thermostat can be stuck close. If you feel the upper hose very hot and the lower hose cooler than normal, it may be a stuck thermostat.

• Make sure the radiator and the overflow bottle are filled to the proper levels.

• Bleed air from the coolant system:This is best done by running the engine with the radiator cap off until you see when the air bubbles stop coming up.

• Check the radiator fans: The easiest way is to turn on the cars air conditioner and turn up the A/C fans. Both radiator fans should come on when the A/C starts

• Make a pressure test by using a pressure testing tool (available at most auto parts stores). Use the pressure shown on your radiator cap. Most cars are 16 PSI or less, so don't exceed that pressure. Replace the radiator cap if it doesn't hold the pressure.

The suggested answer here about a bad electrical ground is a possibility, as gauges and senders do depend on good ground return.

Given that your cooling system was worked on, it is possible you have a big bubble in the coolant system which means it has not got all the capacity it should.

Go park nose up on the steepest hill where the handbrake will hold, let it cool down until you can safely remove the radiator cap, open the heater control fully, and idle the engine until it is at normal operating temp. If you see the level drop, it had a bubble in it.

You might also want to have it tested for combustion gases getting in the coolant via a slightly leaky head gasket.

https://www.youtube.com/watch?v=a3J00_e7fyo