Man, My car is a PITA! Always happens when I'm planing on taking it to the track!

I was having an issue with my car with the N20 on. Turned out I was having a backfire with the N20 on that was caused by a bad cap. The center piece where the coil goes was all destroyed and looked burned. I replaced the cap and rotor, new 2deg colder plugs and it ran great! Ran it with the juice, it was awesome. I emptied that bottle, then got a fill. The car sat for 2 weeks as I was out of town on business. I come back, drive my car & now it backfires again with the use of N20? What gives?

basically stock engine with flowmaster cat back, AFR heads and compu cam 2032. plug gap is .35, timing is @ 6deg advanced.
P l e a s e h e l p

 

i forgot to add, I had my cats gutted last year with no problems. My air tube fell off on one of the cats so I welded a plug there. This was a few days ago before I drove the car again. I just noticed a rattle in my muffler when I pound on it. it's not the hangers, there is definatley something in the muffler. could this cause the backfiring I'm having? Guess what I'm trying to get at is I think something is clogged my muffler.

Also, when slowly rev the car in idle, the car climbs very smooth all the way through the rpm band until I hit 5500rpms. Then, I get a hesitation between 5500-6200rpms as I move the throttle slowly, it finally resolves it's self and quicly climbs to almost 7000rpms. I'm thinking the muffler is clogged, but not sure...any help would be greatly appricated!