I recently purchased a used Trek 2009 Trek 2.1 (with Shimano 105 shifters). However, at first the the derailleur seemed too loose (shifting to a higher gear–releasing tension–was fine). However shifting to a lower gear didn't always work, sometimes I had to go down two, up one because it wouldn't shift with the first tap. So I used the knob on the rear derailleur to add tension to the cable, and even did a high stop/low stop screw calibration. It now works great in the middle of the cassette (10 speed), but when I get closer to the ends, I either have problems shifting up or shifting down. This makes me think that the shifters are off calibration (meaning they aren't changing the tension of the cable quite enough). The 105s should be set for a 10-speed cassette, so I don't see why this would be an issue? Is there any way to change/fix this? Or should I just bring it into a shop?