Ok so i just bought an 04 cavy a couple weeks ago and threw a magnaflow muffler, new spark plugs (ac delco double plats), and a CAI on it. But this darn thing has like a spitting backfire as the rpm's drop (in the lower range) and noticeably shifting between 1st and 2nd gear. Can anyone help me out on how to correct this? Its driving me nuts to say the least

I have a 2003 Sunfire with dc sports ceramic coated headers and a magma flow down pipe catalytic converter and stock exhaust after that. Funny thing is my muffler fell off so you could imagine how loud it is but I experience the same thing as you. When I let off the gas it back fires like crazy. I personally find I sounds pretty cool but I would assume for your situation it would be normal as my car does the same thing. I have no engine codes thrown either. ( next week I have a magma flow catback coming in I'm really excited