For the past few weeks now, my car has been incredibly boggy if I don't gas it up to at least 2500 rpms when I start in first gear. Any ideas at all?

I don't think the throttle cable can fluctuate to cause that, and it's not me just giving it too little gas. It's not every take off either. Maybe 1 in every 5 times. I have to throw the clutch back in because it won't catch at all sometimes. Just keeps putting, barely creeping forward in a jerky fashion until I engage the clutch. Then it takes off fine again.

Think it could maybe be lack of backpressure maybe? I have a stock B20B with a huge custom exhaust. I think it's 2.5" piping with a huge straight through muffler with a 4" tip. I do plan on getting another custom job done, something very similar to stock. Any ideas?