Catrin, are you running the tires that came stock on your Jamis? According to Jamis' website, the bike came with Geax Mezcal. I just read some reviews of that tire on MTBR that consistently complained about bad traction in dry, loose conditions. If those are your tires, you might talk to Jonathan about getting some tires known for a bit more grip (or just replace your rear tire). Yes, you'll encounter more rolling resistance, but I think that's better than losing traction and slipping out.