Unfortunately, I cannot test my project with ASUS B85-K motherboard now, but I have ASRock B85M-DGS based on the same chipset. I tested my Win7PE with ASRock and couldn't reproduce the issue. It doesn't recur whatever Win7PE build I try to boot - with integrated drivers or without. Seems that ASUS B85-K have some bug preventing Win7PE from booting normally (not every time however!). Or the culprit is a piece of hardware - these motherboards may have different controllers and, therefore, require different drivers.