95 Gt 5.0 Running Hot

What you're describing obviously isn't normal operation. Are you sure that the engine is overheating? The 5.0's have TWO temperature sensors: one for the computer and one for the gauge in the dash. If the one for the dash (the CTS, or Coolant Temperature Sender) is bad, you won't get a good reading. If the one for the computer (the ECT, or Engine Coolant Temperature sensor) is bad, the computer won't be able to monitor the temperature and will never turn on the fan.

You can test the ECT if you have or can borrow an OBDI code reader that can read PIDs (live sensor readings). One of those PIDs is the readout from the ECT; if the value is always 0 or bounces around to random values, it's bad and needs to be replaced.

The part where your temperature rises when you first start the car, then drops is a normal thing - it's a sign that the engine has warmed up and the thermostat has opened up. But on my car, the gauge rises to the "O" and drops to the "N". While the factory gauges aren't really accurate, that's pretty far off.

What temperature thermostat did your mechanic put in? It should have been a 195* one, that's the stock one. When the radiator was replaced, did you replace the radiator cap or re-use the old one? Our coolant systems are designed to work under pressure, and if your coolant system doesn't hold 16lbs of pressure the temperature will rise higher and faster than it should. Also, if your coolant hose clamps aren't tight enough the system won't hold 16lbs. You can test the cap and system with a pressure tester kit, your mechanic may have one.