Aluminum heads eh?

I know you probably have this covered but.....

Is it possible that the heads may be warped?
A friends uncle installed used aluminum heads that happened to be warped, and he had unexplanable problems too because it wasn't showing any "head gasket" symptoms either.

Another thing to consider is that you do have a boosted engine.....So your intake tract is pressurized instead of under vacuum like N/A engines. What this means is that a small leak will be magnified because its under a lot more pressure - So if theres a little too much clearance between the ring and wall, your whole block is pressurized and it'll force an escape somewhere... Maybe past the new valve guides?
Do you have just an open vent on the other valve cover?

Sorry if this is all redundant garbage, just trying to help.