My 929 had a similar problem. New spark plugs would have the engine running fine but only for 2,000km at best and then it ran like crap again. Being a K series V6 some of the things I did are not relevant but it ended up being dizzy cap and rotor button but being a V6 it could be the old V6 igniter failure problem which means replacing the dizzy or doing a search on MX6 or Probetalk for 'disty problem' for a cheaper and more creative solution.
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