Thanks all for the input so far - interesting stuff.
Intruder - you're probably right that head/tail winds may explain a large part of the groundspeed fluctuation - what caught me out was probably the fact that the speed seemed to go roughly back to where it was after the air smoothed out (but I guess what you see on skymap is a culmination of contributory factors).
sky9 - I know it's common for pax to over-estimate turb levels. I've flown hundreds of hrs all over the World as pax (civ & mil) and would say this was amongst the roughest I've encountered. I've also read about "severe", and it was definately nothing like that.
BlueEagle - I've heard about the relatively narrow safe speed range you guys can find yourself in according to weight & buffet boundary (also had it explained to me once, back in the good old days when it was possible to get on the jumpseat!) - that was really what prompted my original question.
Cheers all.