i have an 02 SVT, and shortly after i take off to work in the morning, ill downshift from lets say 3rd to 2nd, and it backfires and my exhaust pops untill i put it into neutral. and on occasion, when this has happened. it stalls out, or idols down to nothing then pops back up to above 1k. changed the plugs and wire, still does the saaaame thing. eevery time i start it after it sits for a while. wondering if i can get some help. or if this happens and is normal due to cold start and running conditions.