The first thing I would check is to make sure all six cylinders are running.
Rev it up to 1500 or so and start pulling plug wires (one at a time, of course).The revs should fall off noticeably if the hole is working.
A six runs smoother on 4 or 5 than a type IV does on all 4 and this is the easiest way to find out what's going on there. When you get used to how it runs on six, you'll be able to tell when/if one drops out.
JPIII
|