Am pretty sure that is the problem, HEAT, have seen it on some of the newer LASER units (HP for one) and specifically with card stock. I am a fan of HP for the most part but we did a test with a Laser unit of similar capabilities (made by Brother) and there was no issue, did a clean job on the cardstock. You might want to contact HP service to see if they have any advice or fixes for the problem. Good luck!