Have you disconnected the cable and then run the shifter up and down through the gears while keeping a bit of tension on the cable? If the shifter works ok while doing this, your problem is likely elsewhere. If you still feel the problem, then it probably is the shifter and a bit of lube/cleaner should correct it. At our local co-op, Seafoam DeepCreep is our go-to fix for problem shifters. Sprayed liberally into the shifter and then allowed to sit overnight brings most of them back to proper function.