car has a miss................

my beater car has developed a miss lately, and i have changed the plugs, wires, and checked the cap and rotor. It only misses between 1-2 grand then smoothes out, however when ya flog it it seems down on power. to make things interesting it also has a 94 ls exhaust manifold on it, and i had to lengthen th o2 sensor wire. but i have put 300 miles on it since i swapped manifolds. Any ideas???

Did you reset the ECU after altering the O2 sensor wires (changed impedance!!), and put the 94 manifold on? If not, it’s be worth a shot.
Also maybe try changing fuel filter, air filter, etc… could be any number of things.