1997 Chevy Astro Oil Pressure

The Astro Van has an oil pressure problem when the engine reaches operating temperature. When the engine is cold the oil pressure is approx 50 psi at driving speed (a little lower at idle). When the engine heats up ithe oil pressure is approx 30 psi at driving speed. When the van is stopped at a light and the engine goes to idle the oil pressure drops to 3 to 5 psi (check gauges lite comes on). I have replaced the pressure sending unit and switched to Mobil 1 10-30 synthetic oil with no change. Since I do not have a tack I do not know what the idle speed is. Is there any possible problem other than replacing the oil pump which is $950.00 at the dealer