Random sluggish problems

I have a 1996 mustang, originally a v6, it was swapped with a gt motor and everything needed to be a gt before I bought it. Well anyways, recently I have noticed at times, literally randomly, the car is really sluggish. It seems so slow to rev between 2000-4300 when suddenly at 4300 it seems like the rev limiter kicks in. It will not let me pass that rpm and the acceleration and shifting it just so slow. But yet some days it acts normal. Revs normal and quickly like it should, power all though my rpm range, I can actually hit my redline shy over 6000 rpms, acceleration is great. Before I bought it the guy just had the automatic transmission rebuilt. He says it might be a electrical problem or connection somewhere and I do agree with him as just don't have any idea where possibly? I'm new to the forums so sorry for the huge post. I just want to get it figured out because I can't have fun in my pony more than half the time due to this problem. I've had it start slow and sluggish from say a light and all of a sudden it will be normal and quick again to it being the opposite, starting normal and quick to going absolutely slow and sluggish. I have a small exhaust leak that gets much louder when it sluggish and the throttle response is terrible vs when its acting normal. All help and suggestions are highly appreciated thanks.