I've had the valve cover weep a bit but never enough to affect the running, but actually blowing bits of gasket out? That seriously sounds like the gasket itsn't being clamped into place evenly all around. I know I have had to touch up the phenolic (or whatever material it is) spacer between the throttle body and crankcase as it doesn't stay particularly flat.

If you get the motivation in the future, chuck it back together, give it a couple of runs then pull the back off it and see where the gasket is or isn't seating. Other than that you're sort of stuck with finding a reinforced gasket material.