I was thinking you were relatively new to riding...but you've been a member for two years? Curious...how much do you ride? The reason I ask is that it sounds like it could be a couple of things...one, you could simply need to build endurance, or your fit needs to be adjusted. Or perhaps, a little of both. I know that after taking nearly eight consecutive years off the bike (with only a few rides here and there) it took me a little while to really get comfy on my bike again and build up not only my ability to put in some miles, but also to get my back, shoulder and neck muscles back into the groove as well. One thing, don't be afraid to tinker with your position a little bit at a time, it's a good way to find what works best for you.
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Droping the hamer since '86