Couple of things here, first how low is your voltage
actually going? Second I will guarantee the cobalt gauge is quite a bit more accurate than the sender for the idiot light. Third, 10 psi at idle seems a little on the low side to me.

The cobalt gauge still needs to have a good ground and power supply to work right. That said, it is still way more accurate than a mechanical.

What did you use to seal the threads? Any chance
something got inside the sender?

Final question on the sender, what does it look like? It is large and brass, or chromed on the body?