This is vauge and probably misleading but it reminds me of keeping my Dad's old Allis Chalmer tractor working. It had a magneto whaich had gotten rather weak and the only way to keep it running was to pull out all the spark plug leads so the spark had to jump to the plug. This seems to let the spark 'build up' and helps a weak spark regain some potency. You might try swapping in a new coil to see if it helps. Or a new condensor - those can impact how strong of a spark the coil puts out.