Cooking ballast resisters

Keep cooking ballast resister any help would be apprciated. Bought by 67 dart gt conv new and am still the proud owner. Until last june i had never cooked a resister. I was on my way to a cruise with a 273 .30 over and a 284 cam. This year i found a 67 date code 340. It is now a 418 stroker built with all the goodies. I have had a holley 600 cfm and 750 cfm double pumper. Can anyone tell me why i have cooked 2 resister in less that a week. One thing the resisters are manufactured in mexico