At first I though you might have had a rigging problem, uneven or misattached lines. This may still be a factor. Then again, I looked at your weight, 60 kg (132 lbs). I weigh 90 kg (185 lbs.) and used an Airblast 8.4 about three years ago. I seem to recall that 17 to 20 kts was near the mid range that I liked to use that kite in. I weigh 30 % more than you! I think you were rigged too big. What is your normal comfortable wind range for the AB 8.4?. How uneven was the wind? It sounds fairly gusty, stalling in lulls and honking in gusts.
I just uploaded a post to the fksa.org site on this subject using some nice resources that Detlef put together on kiteforum. I reproduced part of the post below:
"What Size Kite To Rig?"
We all ask ourselves that question, sometimes fairly often. When we are right, it's great, when were off, well it can be a lot of work or perhaps worse if we really screw up. Most kite manufacturers have wind speed-kite size plots to help riders in choosing kite sizes. Definitely examine your manufacturers recommendations when considering what kite size to use.
Detlef has put together some interesting guides relating wind speed to kite size on kiteforum at: http://gallery.kiteforum.com/gallery/albuq10
Check it out. It is a fairly unique and useful presentation on the Internet dealing with kite size vs. wind range. Tell us how it works for you.
His plots have been reproduced below:
Full size CLICK HERE
more at: Here
Be careful out there. Power is relative and seriously unforgiving if you are far off in your kite size selection, weather, riggin, preflighting...etc.. This sport can be far more complicated than people give it credit for sometimes.