What "Brand" spark plugs did you replace the old ones with? AC DELCO? What did you gap them at? When was the last time the Plug Wires were replaced? Also Re-check your spark plugs to make sure they are seated well > but what ever you do Don't Over tighten them. Did you replace your Distributor cap and Rotor?
I just bought a new set of spark plugs for my 4100 > I alway's buy the A/C Delco plugs > they seem to work the best, it's been 3 years since i have changed them so they are well over due. I replaced the plug wires with 8mm type with the Heavy Duty Boots that go over the plug. I am in Complete agreement with Katshot on replacing the Wires with the plugs> I am pretty sure that i had to change the "Coil" on top of the distributor cap 3 years ago as well. I removed my Cap today and cleaned the terminals inside of it > They had carbon deposits on them so i will have to replace the cap and rotor B-4 Winter! 2dfx I would replace the NGK plugs with whatever the car came Factory with> just my opinion.