IF one cylinder keept quitting when you throttle down, there can be a number of reasos: bad glow plug, improperly-set valves, loose head bolts, bad piston/ring/liner fit, leaky valves, bad fuel/air distribution, and so on. The engine needs to be checked carefully. It's possible that the compression isn't quite good enough on the one cylinder because it didn't "come in" during breakin, or was somehow damaged at a previous time, or was never right from the factory. It's not possible to say for sure. The engine can only be gone over very well. Only by checking everything else, do we come to the conclusion that a new ring and liner may be needed.