I had a similar thing, I'd close the throttle, engine braking would kick in but then I'd have little surge for a second or two and then back to engine braking. Turned out he air filter needed changing (it had done 17,500km). I changed it and no more surge.
Not being hampered by any great knowledge on the subject my reasoning goes something like this:
I close the throttle, the engine slows down, the semi clogged air filter lets less air through than it should resulting in a richer mixture, the gremlin box says 'shiver me timbers we're polluting the atmosphere' and opens the throttle a little then everything settles back down to where it should be.
But thats just my theory..
It may be that your sole purpose in life is simply to serve as a warning to others.
2007 DL650 (love it)
1983 BMW R100RS (20 years and 100,000k)
1983 Honda XL185S (chases sheep around the farm)
2007 Yamaha YF400 (feeds the sheep, pulls trailers, great up river beds)
1985 Yamaha YB50 (crossed the Southern Alps on that one)
1972 Velosolex S3800 (a blonde moment, please excuse me)
1975 Honda C50 (the original step thru)
1963 Yamaha YG1
And gone are: Raleigh Runabout (x 2), Yamaha FS1E, Honda CD175, MZ Trophy, Suzuki SP370, Yamaha RD 350, Suzuki GT 750, Triumph 5TA, BSA B40, Honda 400/4, James Cadet, Honda XL 185 (but bought another one), Suzuki YF500?, Honda S90 (which I wish wish wish I still had) and a few more which the grey matter won't release at the present.
When I was a boy my parents said 'you are NOT having a motorcycle'. So I had lots.