Another thought - make sure your bearings and brakes are properly in-tune. I imagine, on flats, you're probably cruising at much less than peak power, so a little bit of extra crunchiness in your BB or wheel bearings, or a little bit of drag on your discs, might go unnoticed from a performance perspective - ie, you can keep up with everyone else with just a little more power. Once you start hitting those half mile climbs, an extra bit of drag will slow you down. Making sure everything is clean, aligned, properly lubricated and adjusted can go a long way. You may even have wheel-rub on the frame. I know that the OEM tire that came on my Lynskey was actually too wide for the frame in that it would rub against the chainstay when I put down some bigger efforts (read: going uphill). Putting a tire on that fit properly saved me 5 mins over my PR (set the previous week) on a 49mi ride.