01 Pontiac Firebird 3.8 loud at the pipes when cranking but becomes quieter as it warms up

Here’s another 3.8 doing what they do best..01 Pontiac firebird, has been missing at cruising speeds gradually increasing with temp. and while putting under load,I noticed when first cranked it’s rather loud back at the pipes and quietens as it warms up, eventually the rpm’s will drop,with it sometimes stalling, then having to let it cool for a few, and thus the cycle starts all over,initially it had a 0420? and I replaced the o2’s and a bunch of other stuff, this fixed the code I guess , it hasn’t come back on, but the problems are worse……how bout it? should I make a boat anchor out of her? or go on and change the cat?