You might also change your position while you are riding such as standing up while pedaling, taking your hands off the bar at times, or tensing and then relaxing different parts of your body. Other than that, also beware of how tightly you may be clutching your handlebars.

Like another poster mentioned, it could be numerous things affecting your comfort. Different grips (I like the Ergons also), handlebar height and sweep, fore/aft/tilt position, saddle height, shorter or longer stem, higher or lower rise, and bar ends all might help.

The bike size you mentioned might be in your range, but it does sound like it could be a little small. I'm 5'9 and commute sometimes on a 19" hardtail. I know your dimensions are different than mine, and bike measurements range from manufacturers, but it might be something you want to look into. The size bikes people prefer differ on there riding situation. It is my understanding that a mountain biker prefers their bike a little smaller than what they would prefer on a road bike.