1989 Olds Delta 88 with 3.8L engine, stalls after driving 20 minutes, 104K miles

Have a 1989 Olds Delta 88 with a 3.8L engine and 104K miles. Car starts and drives fine, however after 20min it will stall out while driving, then becomes hard to start or if running stalls again. Have replaced the following: air filter, fuel filter, spark plugs, cam sensor, crank sensor, and ignition module. Computer has these codes, 41, 42 and 44. Also have a new exhaust system which has resonator in place of cat-converter. The car had same symptoms before new exhaust sys put on. Will gladly 'donate' if advise leading to fixing the problem is given. Thanks, JS