How much are you packing for your trips?
One thing I've noticed about my old Colnago is that it is really flexy when heavily loaded on the rear. At least so it seems.
I recently did a drop bar Hybrid conversion (520 steel), and the beast is rock solid with my panniers... at least up to what I've had in them so far. I haven't gone over 50 pounds yet, I don't think. I'm still doing some experimenting on the Hybrid conversion, and will probably eventually choose some lighter tires.