grrrrrr - flexing rear dropout

[zebedee]

my wife's Giant (bought secondhand) is changing gear of its own accord when the chain is under tension (eg up steep climbs). The problem appears to be the rear drop-out/gear hanger, which is attached in one place only to the frame, such that it can move out of line very slightly, whereas of course the rear cassette stays fixed. There is only one other hole on the hanger, which is where the rear mech cage was, which I threw away as a pointless piece of overweight kak. Surely this wasn't intended to provide any kind of structural integrity to the whole ensemble? If not, what is the problem with it and how do I fix it, as I have upgraded it with some nice LX shifters and rear mech, and on the test bed it shifts perfectly, but on the trail is a pig and is annoying her.

Any ideas what to do?

[zebedee]

its front sus only. Need to double check the tightness of the only bolt that connects it to the frame, haven't had chance to whip the wheel off yet and have a proper look, I guess it could have worked loose, but not sure why. It has new front and rear mech, front crankset, cassette and chain so its an all new set-up, and works fine unless under load. I'm sure there is play in the drop out, so it has to be the mounting bolt - will try tightening this weekend