I would try adjusting your fuser temp.  You can do that in the menu screens.  Your cardstock is thicker that regular paper and the heat from the fuser can't bond the toner fast enough.  Does the print smudge off on regular paper?  If so, you could have a bad fuser. 

It works prfect on normal paper. I found where I can change the fuser setting, but it's choices like plain1, plain2, coated1. coated2... Do you have a suggestion for which setting, or is there something specific for the temp? Thank you so much!

If you haven't tried it already, try the coated1.  That should be the correct temp you need.  I don't know if you plan on using this machine for card stock all the time but after you run your print job set your fuser temp back to plain 1.  You don't want to burn out your fuser.

Hope it helps.