Hell...My boy has been riding a Softail that I wired a front end Onto...no solder at all...just jammed together and heat shrink!!!!!!
I figgered he would want a different front end by now!!!!!!
Better work on that this winter!!!!!

 

I was having a similar problem. After 30 minutes or so the bike would sputter, cough, and backfire. In a short time after that it would foul the plugs (rear mostly). It had a Dyna S and recently I installed a new Dyna 5 ohm coil.

I found an Ultima kit that looks just like the Dyna S on ebay for $59. It came with no instructions, but when I researched it on the web I found that the white and blue wires have opposite uses between Ultima and Dynatek. I hooked up the new Ultima and the 2 rides I have been able to get have been trouble free so far.