I don't think your problem is coming from too much frame flex as Aelle tubing should be quite stiff as I think it comes in thicker guages than most higher line tubing and maybe even unbutted/straight gauge(?). I don't think you can eliminate flex on any frame 100%. they will all flex to some extent when as you say, "dig in" or mash on the pedals. I would check the shifters on your bike if they are not maintaining friction/tension and getting bumped down a gear by any little bit of flexing on the frame. Also, check all your derailleur cable guides and casings to see if there are any areas that might need some lubrication if there is any excessive binding or friction. All those things can cause wht's called "ghost shifting" as you are experiencing.