I received my replacement printer from HP Wednesday morning but couldn't get to it until Thursday morning. I spent about six hours yesterday trying the new one - and the old one - with no success. Finally I sent HP a really scathing email to HP support and got yet another "fix" late last night. (You have to return the printer to HP within three days so was really panicking at this point.) I was just too tired to spend any more time on it last night.
So this morning I tried the new "fix" (believe me, it was really very convoluted and involved installing the drivers for a totally different model printer) and IT WORKED!! and I can now print on card stock. I'm not particularly happy with the way it prints photos but at least I can now print on card stock.
So repacked the replacement printer and it's on its way via UPS.
What a relief!
Tip: If you need a new printer, do NOT buy an HP Officejet Pro 8600. It has some very nice features but there are also some major problems with it. (And not just my specific machine but the inability to print on card stock is a known issue.) I want to stop by Staples (where I purchased it) and tell the salesman about this too.