Gas Gauge Issue - Bit Different Than Most

[LuvNLife]

Ever feel like you're living under a dark cloud? I sure as heck do, with all the issues I've been having recently. Here's the new one.

Stock 305/700R combo in an '87 SWB fleetside. New (less than 20 hours ) POL between-the-rails fuel tank, with new POL sending unit into a Holley red pump. Fuel gauge is a new (less than 10 hours) Summit mounted under the dash.

The sending unit was off about 1/8 tank, which is fine. Today, got my new exhaust installed then did a bunch of errands, and it went from under 3/4 to 1/2, then dipped below 1/2, but above 1/4. I stopped for gas and put in right around 10 gallons, figuring there were about 5-6 still in there. Start it up to drive the last mile home, and notice the gauge is sitting at 1/4. Got home, still at 1/4. Shut if off, turn the key back to On, watched the gauge and it pegged Full, then dropped back to 1/4 again, all in the space of about 1 second. Did this twice more, and same result. I'll be able to get the truck in the air tomorrow, and sure could use some troubleshooting tips on where to begin looking for the problem.

Jerry