Temperature guage

A fluctuating gauge could mean the thermostat is bad............nothing is another story.

With that many miles, it's a good idea to do a coolant flush, hose replacement, and you might as well change the thermostat while your at it.
You can do all of these things yourself for about $75.
Then if you still have problems go with the mechanic's recommendations.
I recommend getting haynes manual#92078 for 00-02 tundra.

The system is fairly simple: there's a sending unit, which is just a temp-sensitive resistor and there's the gauge, which is essentially a volt meter. This is an oversimplification, but you get the point. The sending unit is easy enough to check with an ohm meter. It's rare for a gauge to go bad.

The FSM has instructions on how to troubleshoot this problem.

Subscribers have access to the FSM, I think.

In any event, let's say that it IS the gauge that's bad. Do you really want to dump that kind of money into an older truck? I would encourage you to buy the ScanGaugeII and use it to "replace" your gauge. The ScanGaugueII is a small, electronic device that plugs into your trucks diagnostic port and can read out and display a number of different system parameters. I use one to monitor fuel economy AND as a temp gauge. I can't remember the cost, but it's a lot less than $500.