running like poop!

[tsdrummerboy]

I`m stumped check engine light is on showes code p0300 which is multible cylinder misfire.I`ve changed the plugs and wires last month that fixed the problem for awhile.I put on a set of OBX stainless long tubed headers w/ hi-flow cats(don`t think that should cause anything....) by the way cost me$650.00. But after the install the engine light came back on.The light comes on then starts blinking,then stops,then goes off,then....repeats.Ran a code check again showing the same as written above.It seems when I start to accel the engine runs "ruffer" it even backfires.That`s when the engine light starts to blink.Water temp stays within normal range.I check all the plug wires,they`re on tight.Coils and injectors are, I believe original.I have about 83000 miles on an `03.....Any help/ideas?????
thanks,
tim h.