I would start with the condenser, then move to the coil, then distributor. My boat used to run similar (low on power, then backfire like someone kicked it in chin), turned out to be a combination of condenser and coil (changing one only reduced the symptoms).

Good Luck

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me