I just got a 2001 gt, auto with 35000 miles on it. When you are at full throttle, right before it shifts into 2nd, or sometimes into 3rd, it sounds like it backfires, or somekind of hesitation before it shifts, it does shift though. I replaced the fuel filter and added fuel treatment system to it thinking it might not be getting gas. I was also wondering if it might have bad spark plugs, any ideas?