'70 fuel gauge-weird problem

Stumped..My '70 Duster. Here are some details first. All new tank, sender unit, fuel lines. Gauges sent to G car for restoration, and he converted to solid state. Already did the test for the gauge by grounding out, and yes the gauge goes right to full so we know for a fact the gauge works fine. (and it did even before sending to G Car) First sender unit was one of those bad units I told you guys about already-removed and replaced with a unit that is correct. 100% certain no issue with the sender unit!! Oh and also, YES we have the ground strap installed properly.
This is whats happening: When you turn the ignition ON, but dont fire up the motor the gauge reads perfect. But when you start the motor the fuel gauge drops off to Empty. I'm not sure why, but it seems like the ignition switch cuts out the ground when you turn it all the way forward to start the car. Cant figure why the gauge seems to care when the engine is running?
I had been playing around with the ignition switch a bit, what i was doing is turning it forward enough to where you get a fuel gauge reading, hold there a moment, then quickly engage the starter and back off "ever so slightly" without starting up. At this point, the gauge still shows a fuel level. Then I go ahead and start the car, and the sunnofavitch then seems to work properly. Of course, who the hell wants to go through this all the time????
There are no additional lighting or gauge issues. Been there, done that on this resto. We seemed to have chased all the electrical gremlins away except for this dirty bastard.