VT - are you saying you put a Voltage Regulator (stabilizer) in the fuel gauge circuit ?
Yes.
I used a voltage stabilizer from a volvo (chip style)#1362674 rather than the larger metal base one 1234971 early Voltage Stabilizer, both have the same chip (voltage regulation unit) but the early unit was bulky.
The metal part that the Voltage Stabilizer screws too is the heat sink.
Wires are Red=12V
Black=earth / ground
Yellow-Blue = Voltage Stabilized power.
The early euro cars that ran the original style like the fuel gauge has in the sixty were prone to problem , they either had fuel pumps changed prematurely OR thermostats or dio-ed with overheating . Those were the ones that the owners or tech's noticed that something wasn't correct.
The check was FIRST , make sure the gauge is reading correctly , many would look OK with the key on and engine off (KOEO) , but start it and let it start charging the battery the engine would look like it started to overheat.
Not the case , the fuel gauge was never relied on due to folks either never put fuel in or always drove with a tank full.
The test was a valued resistance factor that replaced the engine coolant temperature sensor , it's "R" value would make the gauge read normal operating temperature on the dash (either straight up or level flat).
If the gauge looked wonky in that test , then a few more quick checks were preformed (Voltage Stabilizer checked if old style & replaced) to voltage supplied to the sender.If they ran needle low , so always showing cold and more fuel than reported on the dash, then those are the engines that really got overheated. if they looked like they were running at the set normal range , they were cooking. I only saw them dead after the fact. You could catch them (Volt -Stab) on rollys if you notice that the temp never made it to normal operating temps,then checked why.
So to get one old expired system out of the way, I updated the voltage regulation by removing the fuel gauge internal one (cut 2 wires ) and installed in an external one. Now any voltage from 8-18 my fuel and coolant needles remain the same level.
VT
PS: Spike Strip, sorry it took so long, just caught this the other night , i needed to find the picture& post the reason.