There could be a kink in the cable housing which is preventing the shifter from returning freely.

Something I've been meaning to try is one of the thinner brands of 26" street tires. You should be able to use a 1.75" Panaracer Pasela or a 2.125 Schwalbe Manhattan at a lower pressure without too much drag unlike knobbies or street slicks with a thick rubber tread.