is it an auto? if so the slush box will definatly be one of the problems. Aside from that, if its a serious dead spot, not just your motor not being able to get outta its own way, It could be any number of things. Fuel delivery, timing, I even saw a car once, with a bare wire that would brush up against the fender well and shut the car off when you turned right..... try trouble shooting that.
Your sig says no cats, and aftermarket mufflers, if you have too wide open of an exhust, the gases wont pull themselves from the bore and your car wont acclerate either.