Missing at Low RPMs or when accelerate fast

I have a 1998 Mustang GT, when I bog down the engine instead of downshifting from third to second it will miss very badly and it never did before. It will also do the same thing from a start if I try to accelerate fast, but if I left off of it I can work through it. It is worst under hard acceleration. I have replaced plugs, wires(twice), and coil packs. I replaced the wires once at the dealer because one was broke. I then started to get this missing problem a year or so later. My mechanic said it was because the dealer did not put in any dielectric grease in the plug wires. He put the grease in and it was great for about five months and then it started again. He said I should replace the wires because they were fried because they were arching. I replaced them with larger ones and loaded them up with the dielectric grease. That was a little over a month ago and it is doing the same thing again. The only thing I know I did not do was gap the plugs when they were put in the car. However each time the dielectric grease has been added the problem went away. What can I do to get rid of this problem?