Loss of oil pressure as engine warms

My 1999 Ford Taurus, 3.0 liter engine, 118,500 miles was put in the shop for a cooling problem, the next day I drive to work only to break down again this time with a blown head gasket. The mechanic replaces the heads and head gasket. I drive back to work the next morning, and start having oil pressure problems about 80 miles into an 85 mile trip. I take it to a seperate mechanic who takes down the oil pan, cleans it out and replaces the sending unit. He said the first mechanic left all kinds of sludge and gasketing material down in the engine which clogged the sending unit and pickup tube up. He also said the main bearings are now weak and a direct result of this. I am still getting low oil pressure in the engine especially after it gets warm. Before it warms up I have pressure at 1000 rpm but after it warms my oil pressure light comes on at 1700 rpm. Can the oil pump itself cause my problem? Is there any way to fix this without rebuilding the engine? I travel approx. 170 miles round trip a day & this was my only reliable vehicle before the trouble and only way to work.