I did my own tuneup on a 99 suburban 454 7.4L big block with 250k miles, replaced plugs, wires, distributor cap, rotor and afterwards it was missing badly and backfiring. I couldn't figure it out why so first took it to another shop and they replaced the distributor for which the teeth were worn, however it was still missing badly and they said it may need a new engine. I Took it to extreme auto. It took a while to diagnose but they found that the brand new distributor cap I bought (oem brand) was defective. (I discovered later that two plug connectors for cylinders 3 & 5 were shorted internally, a very rare problem) so 2 cylinders were not even firing or firing at the wrong time. They replaced the cap and now the car is back to running good. I wished I could have found this problem myself but I was in a bind and they really saved me. The bill was reasonable for the time spent.