Excessive crankcase pressure will do this.
You need to check a few things to make sure there is not a problem with the distributor itself, the bushings for one like Ray stated, but more importantly, you have to realize that there is actual oil pressure on the distributor shaft at the very bottom of the housing, so if crankcase pressure is excessive it will force oil straight up to the centrifugal advance mechanism and sling the stuff everywhere.
I would start with proper venting of the engine crankcase pressure and going from there...These are tried and true designs and this is the only thing I can think of. Big vents on the valve covers are a must have.
Good luck
GT
