Hi, I have a 1999 civic ex(1.6 SOHC V-TEC) with 45,000 miles on it. My car runs beautifully when it is cold. When I say cold I am not refering to sitting over night but I mean I have gone for a drive and parked the car and 1-2 hrs later I jump in the car again and it runs amazing. After a while once the car gets the chance to warm up, it's starts to lack power. I hit the accelerator and it just doesn't move nearly as well as it does when it is cold. It like hesitates feels like you got a really heavy load in the car. Even if I floor it or drop the gear, the rpm rises and the engine roars, but the acceleration is not as quick or smooth as it should be through out the entire rpm powerband.
I recently purchased a performance muffler for my car, now it wasn't brand new, slightly used but in excellent condition. It wasn't a universal one, it was actually made for my car. Now the way these mufflers work (I think, because it does it with my car) the warmer your car gets the louder the sound of the muffler gets. Now this goes hand in hand with the problem I am having. AS THE ENGINE GETS WARMER, THE MUFFLER GETS LOUDER, AND AS THE MUFFLER GETS LOUDER THE CAR STARTS TO HESITATE. It could be that the muffler and the problem I am having are not related and are just seperately reacting to the operating temperature of the vehicle.

I really really appreciate any suggestions as to what the problem could be.