Does it idle fine, then run like poo when you step on the gas?
Happened to me when I did a tuneup once, looong ago, when I first acquired the truck. It had sat for several years while my dads health was failing, so I bought new cap, rotor, points, plugs and wires for it. After the tune up, it fired perfectly, and idled just fine.
Problem was it had no power, and stumbled and sounded like it was backfiring as soon as it got throttle. Ended up being #7 and #8 plug wires reversed, and yes, I had double checked it, even triple checked it on the side of the road, and still didn't catch.
One more thing to check, is your distributor clamp loose by chance? They CAN loosen by themselves over time, and even though they wont move, you might move it inadvertantly when you mess around with the cap/plug wires. Shouldn't make that much of a difference if you didn't notice it happening, but it's worth a look anyways.
) But then again I might be making that memory up because I'm about to strangle my truck and need a part to blame 
