1989 Cadillac Deville Backfiring/very rough idle.

I own an 89 Deville. I have replaced the fuel pump, strainer, filter. The fuel pressure regulator, distributor cap, rotor, pickup coil, all spark plugs and wires. Battery is new, alternator has been tested and passed. Egr valve moves freely. It will run fine for about 15-20 minutes then start acting like its starving for fuel. It will sputter then surge and begin to backfire. I have read many of your messages to others with similar problems. The only things left I can think of are the MAP sensor and the ignition control module in the distributor. When I do the computer diagnostic check the codes I get are E21 and E 28. I'm just at the point where I'm throwing money at it. If you have any suggestions or advice on how to proceed from here I would be very greatful.