Electronic oil pressure sender

[LUV2XLR8]

I have a small block LS that I'm utilizing the standard oil pressure port at the firewall side of the valley cover. Engine cold -- oil pressure is around -+65-70lbs and when warm (185-195*) it hovers in the 55-60lbs range. When the car idles for about 15-20 minutes the oil pressure will slowly drop to zero. So thinking it was a bad sensor, I changed from a single feed unit to a unit that required a ground strap. Installed it and same thing. For sanity check, I changed out the stock GM oil pump and switched to a Melling high volume pump. Same thing. Sitting there stumped, I removed the electric pressure sensor to a standard sun-pro manual gauge and boom; I never lost oil pressure. Ran at operating temp for 30 or so minutes with the gauge showing roughly 60lbs the entire time. Figured to try a cheap 100PSI EBAY unit and to no avail, same result, slowly crept back down to zero lbs of pressure. I probed the pressure sensor and while it was dropping it was reading about 120*F.... with the engine and oil at operating temp, I flipped back to a previous pressure sensor that was cold (room temp.) and again had oil pressure, 55-60lbs just like normal... As the swapped unit climbed to about 118*F, again the sensor started to loose pressure until bottoming out at zero. The gauge is mated to a brass LSX gauge adapter, no thread tape only a crush washer. I am running a low profile Summit LS oil pan that has a pressure port that I can tap into, but, at that point I would have to extend the pressure wire from the digital gauge and clearance the $1K Doug's headers. Am I the only one who has experienced this issue?,