98 Tahoe 5.7L will hardly run cold,backfires, stutters, rpms drop immediately then pick right back up. Get it out on the freeway and engine warms up, runs like a sewing machine !

Set 3 codes, passenger upstream and downstream o2 sensors and EGR
valve. Replaced upstream no improvement. Already had crankshaft position
sensor replaced. Had to have pasock disabled in order to even get it cranked
when the battery went dead while in the shop for a month or so. I’m at my
wits end, next move is replace egr valve, but I understand engine temp don’t
effect their performance. Someone installed Flowmaster muffler on it and
relocated the downstream o2 sensor in front of it. My understanding is that
these emission control items will not make an engine backfire, go dead, and
try to self destruct like this. Will disconnecting the negative battery terminal
for one hour to clear the codes help any at all ? Any ideas about what could
be the problem that isn’t setting a code ?