Not just up, but down too. If it works both ways, then probably your DR needs adjusting and maybe the cable slipped in the hold fast on the DR.

If you are just itching to order a new shifter, then order it. But remember you still are going to have to do all of the things and more that you'd do to simply replace and adjust the cable. And I don't know that you've eliminated the derailleur from being the culprit.