I had something similar happen to me and, with 4 cylinders, it was harder to identify. I filled a small squirt bottle with water and put it in my tank bag. The next time it lost power, I gave it a minute and then pulled over. I squirted each of the header tubes, a few inches out of the head, and you can clearly see what is firing and what is not. The firing cylinder will evaporate the water before it can touch the metal. A dead cylinder will allow the pipe to get wet and take a few seconds for the water to evaporate. Once you know which cylinder is the problem, you are almost there!