Most folks don't know they have blown head gaskets due to power loss; they notice the coolant puking and the high engine temps... You can continue to drive with blown HG's, although you should take it easy. BTW, the heads were prone to warpage (depending on how bad the gaskets and how hot the block was, and they recently updated the spec to .002. Do you know if they mic'd the heads and what they were at the time the HG's were replaced? It sounds like one or both of your heads are warped or your HG's are bad (again). That would be a pricey fix without a drivetrain warranty. Let us know what they say...