I had a 97' 4.2l V6 that I was going crazy with the similar problem after taking it to the dealer several times I decided to do my own troubleshooting. With the truck running I took a pair of plastic pliers and pulled spark plug wires off one at a time and discovered that the pack was bad. I know you said that you replaced the packs 60k ago but figure it is worth a shot. I find that sometimes it is best to forget what has been done and start fresh on stubborn problems.