I'm the same height as you and ride a 22" x 24.5" I would say yours could be causing you problems depending on the specifics.

I find that any problems with ride I have can be pretty much cured by tire choice. I have a bad back etc... but am comfortable with 1.5" slick at 90 lbs. You could check out Bob Brown's site to check out his numbers for why Al is not actually rougher ride, and he's a steel builder. I've been comfy on both Canondale Al and various steel bikes, I don't really think the differences are that significant. At one time they used to sell aluminum as being a softer ride, presumably because it's a softer metal. I'm not saying that's true, but there is only so soft that a well designed double triangle truss with a vertical member up your a$$ can be.