I have a question about fuel gauge failure in my 1966 122.

On cold days (below about 60 degrees), my gauge works just fine. It's as accurate as any I've seen in a 40 year old car. Sure, it slops around as I drive, and it flickers up and down from time to time, but with patience and an appreciation of the quirky, it gives a reasonable approximation of how much gas is left in the tank.

On any day over 60 degrees (F), or if the car is parked in the sun, the gauge begins to show that the tank is significantly emptier than it really is. Instead of showing 3/4 full (when the tank really is 3/4 full), it will read 1/4 full.

On really hot days (over 80 F), the gauge will simply show the tank as empty, and the needle does not move at all. But the next morning, after it cools down, it's back to normal (or at least semi-normal).

So here's the question: Where is the source of the problem? As I understand it, when the tank sensor fails, the guage needle usually flickers up and down completely out of control until one day it dies completely. I don't want to go to the time and expense of replacing the tank sensor only to find out it's some bizarre heat-sensitive electrical failure elsewhere in the system.

Thanks in advance!
--
Dennis