1995 Pontiac Grand Am Temp Gauge unreliable

Within a few minutes of starting car, temp gauge shows it in the red, then after a few more minutes it goes back to normal. Then back in the red, then below normal. The water pump, thermostat, temp. Sending unit, gooseneck, and radiator have all been replaced. It's a quad 4. Some other mechanics(the ones wanting to make a lot of money off me), tell me it's a blown head gasket. However, I'm not losing any coolant, nor do I have any water in my oil, or vice versa. Even when temp gauge shows car to be running at 280 degrees, coolant is only luke warm, and you can lay your hand on top of intake cover. Also there is no water, or coolant escaping out of tailpipe. I've checked all the fuses that I know of, and they all seem to be in good condition. Is there a control module behind the temp gauge that could be faulity? Or is there something else that's the culprit? Thank you for your attention in this matter.