Bought my Model D 10/06, and have been loving it ever since (~900K to date). I've done mostly interval training w/ the IP, and only began rowing for longer than 30min at a single stretch about 8 weeks ago, when I began doing 10Ks and the occasional 60min row. After a few of these longer rows, I developed an incredibly sharp pain in the butt area, but it's a weird pain (see below). There have been two episodes thus far. The first about 8 weeks ago, after which I saw the doctor due to its severity. Took it easy for 3 weeks thereafter, and only recently began rowing again. Rowed in shorter (<=20min) intervals until last night when I tried a 10K uninterrupted (at a very moderate pace), and the pain returned.
[Aside: A detailed description is below and I'd appreciate any guidance, but please note I've already seen a doctor about this issue. He did an exam, took x-rays of my whole lower back & mid section, and found nothing physically wrong.]
The pain seems to only occur when I initially exert or release pressure on the butt area. In other words, it hurts immensely for a couple of seconds when I first sit and then again when I release the pressure by standing up. However, walking around and sitting in one position feel completely normal. The only other time I the pain occurs (and also when it's at its worst extreme) is when there is pressure in that area of my body due to using the bathroom (happens during either manner of "business"). If left alone (i.e., no rowing) the pain lessens over a period of 4-5 days, and disappears entirely after about a week.
Sounds like a stress injury, right? I initially thought it was something w/ my tail bone, so bought an extra seat pad after the first episode. That didn't solve it. I then thought my technique might be faulty in a way that only causes problems on longer duration rows. However I've been very diligent about keeping proper form (unfortunately don't have a video recording device to prove it), and anyway wouldn't improper technique have hurt me after rowing several 15-20min intervals just as much as it would have rowing 45-60min at once? Next, the dreadful "kidney stones" idea passed through my head, but I'm relatively young and eat well (26, male, 6'3", 187lbs) and the doctor said that wasn't it either (**phew**). Plus, that kind of pain (as well as any due to a non-rowing genuine medical problem) wouldn't go away on it's own after a week's rest.
This pain is terrible, and I'd really like to figure out a way to avoid it as I've fallen in love with my C2 and can't bear the thought of having to stop using it! I figure there's got to be a solution, as thousands of people all over the world are rowing lengthy durations comfortably with few if any physical consequences (other than improved fitness).

Anyone ever have a similar problem or ideas about what to try next time I'm feeling better? THANKS!