First things first, do you actually have a problem:
You indicate that it will slowly heat up: what temp does it heat up to? Do you have a real mechanical gauge or an infrared heat gun to check the actual temp? Never trust the factory electrical gauge.
Remember, some of those so-called NOS temp sending units are way off when it comes to accuracy. Most aftermarket ones are crap, too.
I had one supposed resto sending unit that made the gauge read 220 when the car was at 160 - on my 72 T/A. Don't even get me started on the Chrysler combination temp gauge and voltmeter that moves up with the rpms and falls when you let off the gas. [img]<<GRAEMLIN_URL>>/rolleyes.gif[/img]
A vintage sender is the best bet for accuracy. Second best was the Wells TU5.
Here's some info when I tested a few senders:
http://forums.performanceyears.com/f...070&page=2