Difficult. The original gauge setup, the limiter DID pulse, just like a flasher, only faster. You did not see it because the gauge is what is called "heavily damped." Meaning, it takes a short period of time to react and you don't see the pulsing, even though it is there.
Seems to me this could be instability in the limiter, BUT
THE TEMP GAUGE (and oil gauge in a Ralleye cluster) all use the same limiter voltage. So if your temp gauge works OK, it has got to be a defective fuel gauge unit.
Only way I could see the sender causing pulsing is, if the fuel is sloshing, AND the gauge is not damped enough
