1998 jeep grand cherokee larado 4.0

i have a check engine light on and i cant get rid of it . I took it to auto zone and they did a scan on it and it was 3 codes.
1 P0420 2. PO351 3. PO108
ok oxygen sensor, map sensor, and coil i replaced all 3 and then went back and he reset the codes and i was ok for 1 day and they r back.
then i went back up to auto zone and he scaned it and bam they r back then he said just drive it for 2 days maybe computer will reset this was december 2014 i am lost here . so i called a dealership they said u might need a new computer at 750 bucks . Help and oh by the way the jeep has a bad stalling problem with a bad moldy foul smell when he stalls i let it sit a while and it runs badly//