You say you are following the facory torque procedures, but whether or not you are aware there are two procedures for the heads based on the head bolt size.

Early engines: M12 x 1.5mm head bolts
Later engines: M11 x 1.5mm head bolts

The torque down procedure is very different bewteen the two.

I will not type out the lengthy tightening procedures as you may already be aware of the above. If you need the procedure post back here with the size of head bolts you have.