[ItsOnlyMoney]

While tuning on mine we had the same problem as you describe. Turned out my engine builder had put too cold of spark plugs in ( which is probably good on a new engine being broken in on the dyno ). It dyno'd great, but over time would foul out the plugs. We went up two heat ratings on the spark plugs and all is good. We used a chassis dyno to get the air/fuel correct from idle to 6000 rpm.

David