Kneel down, face Detroit, and pray for a vacuum leak.
What's wrong with an 85 Cadillac with low mileage that misses, shakes, and slowly stalls at idle. At 20 mph, it still misses and shakes. I've replaced the plugs, wires, and distributor cap.
TOM: Well, Willy, the good news is that this problem ought to be fairly easy to diagnose. It sounds like you have several cylinders that aren't firing. A good mechanic ought to be able to figure out why with a minimal amount of detective work.
RAY: The bad news is that you've already eliminated most of the cheap solutions. The plugs, wires, and distributor cap are what we would have tried first. Those are the most common--and least expensive--causes of non-firing cylinders.
TOM: The only other inexpensive thing that could cause this would be a massive vacuum leak. A good mechanic can check for a leak by listening or by actually measuring the vacuum at idle.
RAY: From this point on it gets more expensive. If your mechanic finds gas fouled spark plugs, that could mean your fuel injectors are bad. Injectors are expensive--hundreds of dollars.
TOM: Even worse, if a compression test reveals no compression in several cylinders, you could be a candidate for a valve job. That could cost as much as 1000 dollars!
RAY: So do what any sane person in your situation would do, Willy; Kneel down, face Detroit, and pray for a vacuum leak.