Well to day I noticed that at about 5500 or higher my engine backfires once. It runs perfectly fine and drives perfectly fine. Even after backfiring. What the hell is going on.

like a gunshot or like what the srt-4s exhaust does???

eclipse GONE....02 JETTA 1.8T chip and intake, bailey DV and N75 H valve....

http://www.cardomain.com/memberpage/472859

I have never heard the srt-4 exhaust. It does sound like a gun shot though.

check or change your plugs and make sure they are gap properly

Check out your wires too, are they old?

1999 Eclipse GST 4G63

I hear back fires at Drag strips all the time especially when waiting at the tree. Even I back fired, I didn't worry. Just like everyone said, it doesn't hurt to check for anything wrong.

When is the money spending going to stop on this fucking car?!

I have a set of msd plugs and coil. Both less then a year old. I use the ngk plugs I usually change them every year and it is about their time. What gap do u think I try with the new mods. Cause it might need to be closer. I am running on stock gap But I noticed sometimes there seems to be a big lag in power when I mash the pedal. Like it didn't burn it off right.

Before changing the plugs,
try running some of the mopar combustion chamber cleaner.
Could be the valves not seating properly.
I had backfire in my intake and exhaust.
It fixed both.