well, Becs, I've considered your considerable bust and I don't think that's where the problem lies. I think you're a little out of condition, you've tweaked something, your position on the saddle is a little awry and you're not as 'still' as you might be. I'd work out what the best position for your feet (which might just involve moving the cleats forward a smidge) and get yourself some no-float cleats and stick with those. And, if you have cleats that tell you precisely where your feet are you can move more of your weight on to your feet and your hands which will give you the freedom of standing when you need to put in a burst of power.
That said, if you go down to Brixton Cycles they'll give you good advice. They fit bikes to women and not the other way around. That, as we both know, is a virtue.