My GT-40's had the same problem and I had to get new valve springs...I remember it was just above the 5,000 rpm range that I had the problem. It sorta felt like a rev limiter...almost like someone had changed rev limiter from 6250 to around 5200...I'd check it out...always could be something else but has me wondering since that kindof acts like mine did and I had to replace mine.
Red '86 GT w/T-Tops, Cervini-Mach I Hood, Cervini Fiberglass hatch, Cervini fiberglass cobra wing, FMS 302 SVO Shortblock,Explorer Cam,Crane 1.7 Rollers,FMS-Double Roller timing chain, FMS 65mm Throttlebody, Cobra Intake Upper/Lower, FMS GT-40 ported iron heads, Dynamax exhaust, FMS Aluminum Adj.quadrant, FMS SVO T5 World Class, Hurst w/aluminum(line lock) T-handle, FMS Aluminum Driveshaft, FMS 3.73 Gears, M-1007-C58's (FMS Chrome cobra R's) all way around on BF 245-45-17's. 5 lug 13" front 12" rear baer system, MM Adj. lower control arms, MM full length subframes, Battle boxes.
DFI Generation VII w/wideband
2 Bar MAP (added 5/31/02)
T3/T4 Twin Turbos, MSD 6A, Seiman Deka Low Impedence 55# Injectors, Walbro 255 lph GSS340 pump