I had a similar problem on my A10 Super Rocket.
Spent a lot of time and effort on preparig the gasket surfaces, cleaning out threads, counter sinking the holes around the threads etc.
After my first ride (engine running sweet) i noted that there was oil bubbling out of the barrel base studs and the head gasket.
Also had a serious wet sumping problem. (see my entry under A10 wet sumping)
Stripped the motor to discover that i had misssed the anti-flow back ball when i rebuilt the motor.
The ball is directly behind the delivery side of the oil pump, accessed from inside the crank case, next to the timing side main bearing.
It was out completely and the oil pump was delivering direct into the crank case! OOps!
(i had not stripped the motor, got it as a basket case). Note that the Haynes manual i was using makes no reference at all to this component in the strip down restoration or assembly procedures.
(except a small pic on the parts listing). Hence i missed this first time round! (yes guys, i felt like a fool when i eventually worked this out).
So the result was excessive oil in the crank case and over pressuringing of the crank case forcing oil up and out of my well preapres gasket surfaces.
I dont know exactly what has caused your problem, but it is worth checking that the crank case breather is not blocked and that the crank case is not pressurising on the down stroke.
If you run the motor with the oil tank cap off and then stop the motor, you may well hear a hissing sound int the oil tank, if the sump is holding pressure!
That may well be a possibel cause!