98 Pontiac gtp running very rich

took car in for new cat, old one wore out leaking at collector and rattling in side, they
did some gas test and said have to fix rich condition before can install new cat. I new it
was running rich and had both o2 sensors replaced earlier. Also had replaced  injectors
earlier with bargain set bought on line. It has a hard to start problem except 1st thing in
morning.and intermittent up and down idle. air flow sensor replaced but still does it. It
also has a new maf sensor. My ECM is performance flashed and about 8 yrs old.
What am i missing here thks Andy