Is there a possibility that too much pressure is building up in the rocker cover and hence causing it to leak. I had a chonda that had blocked oil passages that was doing a similar thing. i ended up stoking the passages back towards the crankcase with some 1.5mm tie wire, spraying some carby cleaner through and blowing with compressed air and all was good.
[Linked Image]
[Linked Image]
On my particular example i've circled the passages i'm referring to in yellow.