Tom........unfortunately, neither the original 6-volt nor the modern 12-volt squarecan coil are as reliable as we'd like them to be. BOTH need the "infamous ballast resistor". Your current 12V conversion is using the WRONG 12-to-6V converting resistor. Most tractor parts stores do NOT sell the correct value.
I recommend a 2.5-ohm, 50-watt 12-to-6V converting resistor made from Radio Shack (2ea 5-ohm resistors in parallel) for use with the original 6V squarecan coil. It should be intuitively obvious, you do NOT use a 12-to-6V converting resistor with a 12V coil as you will have weak sparkies.
BTW, it takes about 1-hr of operation for the internal insulative tar to melt and cause weak sparkies. .......Dell, yer self-appointed sparkie-meister who knows 8-ways to convert to 12V and they all work the first time