Q: Stutters under acceleration or load, fine at idle

working with a 2001 chevy impala LS, 3800 series ii engine - began to really run oddly when accelerating or under any kind of load. cleaned MAF sensor, replaced air filter, front 3 plugs and wires (those back ones are tough!), checked & cleaned EGR valve and it was not blocked but i put on new gasket, replaced PCV valve. discovered that duct from filter to throttle body was not tight. has check engine light with a p0446, does not seem relevant. cel blinked a few times while stuttering, no new code

also, is loud, my resonator is rusting out, wonder if that or converter could be an issue. suggestions on where to look next?

oh, has about 185k miles on it.

My car has 18000 miles.
My car has an automatic transmission.