I had a similar issue starting to happen now that I'm riding 20+ miles each ride. Bike fit, stem too long. Beetl (a poster here) sent me a shorter, angled stem and I had it installed today (the old stem was a Sterling without a 2 bolt cap so the bars had to be unwrapped) Rode about 15 miles tonight and already i can feel a huge difference. More upright, less pressure on the hands, feel much better in the drops, etc.

Thanks, Beetl!