I'm betting based off of your descriptions and what I hear that's a spark/timing knock. Pull the distributor cap off, remove the rotor and I bet you find the advance weights, springs, pins and bushings are completely south of cheese. I believe that year still uses that style of distributor and isn't electronic. If it is a mechanical/vacuum advance type distributor, what happens is when its revved up or idling high, centrifugal force throws the weights out to a "normal" position for spark/timing advance but as it idles back down the weights come in and due to everything being worn out it overthrows them too far the other way and then the timing goes out of whack and makes it sound like dieseling/knocking. I've also see weights get stuck open on pins and bushings that are worn out as well. I didn't read through the other posts, but that's one of the places where i'd look and then invest in a tune up and possibly a new distributor and go from there.
