1998 Chevy Lumina PO420

Car first started missing, not backfiring, when I stepped on accelartor harder it would almost die, make a popping noise, bilge black smoke out the exhuast and then chug, sometimes ok after this, sometimes not. Replaced fuel pump and filter, good for 2 days. Replaced crank sensors, good for 20 miles, got code po420, had cat replaced, good for 2 days, had ecm replaced, good for 3 days, then started to miss, had mechanic ck again, replaced plug #3, good for 1 day, placed all plugs. Drove 140 miles 70 each way to work, heard loud screaming noise from fuel pump, filled tank from 1/2, was good, next morning drove 65 mile ok, went to return home and stalled after 1/2 mile, restated and drove hom. Next morning, would turn over but not start, sounded like no spark. Next night started and drove to mechanic again. Found nothing wrong. Drove car for 2 days, service engine light came on, had advance auto read and found po420. I replaced ignition baord under the coils and was fine. Went to start next morning and will not start again, souynds like no spark but smells of fuel. When I call the mechanic again on monday, this will be the second time it did this, but the code is there now. 1400.00 and no answers, it drove perfect everytime they have it and then after a 65 mile, each way trip, and sits over night it wont start, any ideas, im stumped and goiing broke. Thanks much