Centaurus,
If you lean the mixture controls to 68% fuel flow on the flow gauges then I would suggest that prior to leaning you should have consulted the AFM to determine the correct power setting for your pressure altitude and temperature. In other words the correct MP/RPM for 68% power.
Sounds to me like you had a higher power setting than 68% selected on the MP/RPM! If you had say, 75% set, simply leaning the mixtures to 68% will result in running the engine on the lean side of peak with just the symptoms you describe. Leaning to 55% would definitely result in total power loss with higher power set on throttles/RPM. Not good for the engines old boy!