I have a new bug to figure out on my 91 GT. For the last couple weeks when I crank 'er up to 5500 the car falls off flat on its face and starts to backfire and sputter. I have new AFR 165's and the Holley Systemax upper and lower intake on a stock bottom end. Not boosted or NOS'd. Do I have a fuel delivery problem? I also installed an adjustable regulator and a 255 lph pump. Or possibly a broken valve spring? The car drives perfectly under 5500 but every time, right @ 5500 it falls off. I installed the intake and heads almost a year ago and this is the first problem.