My guess would be the valve cover baffle under the breather, isn't keeping the oil from splashing up the cap. That's why most race car motors run an extended pipe from the valve cover to the breather cap, for high rpms. Make sure there is no direct oil splash, able to get to the breather cap. The reason it doesn't do it until you turn the motor hard, is beacuse the oil isn't there in a large quanity, at lower rpms. I can't explain why the left blows and the right doesn't, except there may be more oil, coming off the pushrods on the left side.
