I've had exactly the same thing with my VW R36. I got it in 2008 and particularly when coming out of the car park at the office I'd start off into the traffic, ease of on the accelerator and then need it to go again and .... Nothing, for what seemed like forever (especially if a truck was closing the gap on me) but was only a second or three and then pick up again.
I haven't really noticed it for the past 4 years ... It either tuned itself out, sorted itself with software updates/resets or I got used to driving around it.
It was certainly worse when I left foot braked or quickly came off the brake, but not only then.
Magically I don't have it happen, or just don't notice it

, now
My reckoning is that it's part of the VAG control system associated with the DSG and motor interaction.
Oh yeh, left foot braking is not meant to work on the R36, but with good country driving you can use it, although to extreme. I had been underthe impression that the RS4, being a classier more sporting drive than the R36 allowed left foot braking. From reading here, and reading APR flash upgrades that reintroduce the ability, it seems like I'll have the same interaction as the R36.
I have been really happy with the R36 and the way it's done things, which is one of the reasons why the C63 doesn't quite suit me and why the RS4 does!
So, I'll still be very happy when my RS4 arrives late July.
