Engine rev surge

Can anyone kindly help please.

Heavy traffic. Done probably a mile at less than walking pace. 

I'm in D2 on the DSG controlling speed of creep on the foot brake.

Suddenly with no warning engine revs rise fast to 3200. Hold for 1 or 2 seconds then drop to normal.

These revs have no effect on road speed or I would have smashed into car(s) in front. I know how fast this thing can accelerate.

In fact it is very unruffled and feels fully under control all round. Like the car or just the gearbox is pretending not to notice anything unusual at all.

It has done this three times (all in crawl) in the last three months.

Dealer says never heard of it but "lets get it in and see what we can find"--like my wallet.

Thanks.