1st cylinder missfire

I have a '96 Corolla with 115,000 miles on it. The engine light is on and when i have it read it says there is a 1st cylinder misfire. I have replaced the plugs, wires, cap and rotor with no improvement. Any suggestions? Could it be the distributer? I have had the engine light cleared and it will stay off for about two weeks at a time but then comes back on. The engine idols really rough at times but not all the time, just occasionally when at a stoplight or something. I dont notice any problems while driving at higher rpms or higher speeds for that matter but not to say that isnt happening also. Any help please... Thanks
Oh and the engine was rebuilt about 20000 miles ago from a dealer, the engine light was apparently on when my father-in-law picked up the car and they told him that it was due to the altitude...as the car is normally driven at 1100 feet but was rebuilt at 5500 feet, and that the light would just turn itself off... i thought it was a load of crap too!

What does the #1 spark plug look like? Is it black and sooty? Wet? Oil contaminated?

okay, so my mistake... it is actually the 2nd cylinder that is misfiring. It had been about a month since i replaced the cap, rotor, wires and plugs so i decided to check everything out. Well it turns out now that i dont even have a spark at the end of the wire for the 2nd cylinder! So what could that mean? I will pull out the plug tomm morning and tell you what it looks like then...

the spark plug looks fine. And after removing the distributer and putting it back on there is now a spark in all four wires. But the engine still idols really rough and there still seems to be a misfire somewhere.... thoughts?

go to the gas station...buy 5 dollars worth of gas...pour on your car and ignite...stand back and light a cigarette with the inferno...jokes man...dont get offended...im stumped