Has all the hallmarks of lee activity; perhaps the temperature/lapse rate and wind direction were all just right to generate standing waves at a natural frequency. You might have taken off into gently rising air, then at 1000 ft entered gently decending air?
A similar, but much more unpleasant encounter happened to me many years ago in a heavy 4 jet leaving Canada. It was almost uncontrollable and there was little warning (nothing forecast). We lost a lot of radios and nav kit; fortunately most of it recovered after re-racking in the avionics bay.