Tim, you could have a slightly cracked or separating pedal with no sign of play unless you really manhandle it. Same with the BB; it will usually be at least a bit loose if there is anything wrong with the bearings, but you might have to exert a lot of force to notice anything if the problem is not too severe. With either a busted pedal or a spent BB, you're certainly going to feel it more when climbing.

10 Wheels, with everything you replaced, seems you've eliminated any possible culprit on the bike. Have you checked your feet? No joke, had a similar problem myself once: after dismounting from my rickety steed and taking off my shoes, noticed a clicking sound eminating from down below with every step I took — That problem eventually resolved itself but my pedals were still shot.