My tranny did that for about a week and eventually all forward gears went out. Best of luck maybe you can find a problem, I ended up just replacing mine.
No engine/performance mods (yet). Mach 460 sound system, 2 12" alpine subwoofers in a custom box w/ 1800watt kenwood amplifier. Smoked halo headlights, smoked sequential taillights, black bullitt staggered rims, racing stripes, and plastidipped spoiler. Future exterior mods - Chin spoiler, black fuel door, and demon fender flares (plastidipped).