'99 Tiptronic Surging?

[PeteBrown]

I recently took a look at a '99 cab with a Tiptronic and discovered that the car surged noticeably in 5th gear when the engine was below 2,000 rpm. The engine seems to hunt between 1,400 rpm and 1,800 rpm when in top gear but only going 40-50 mph. When moving at highway speeds with the the engine above 2k, there is no surging. Any idea what the source of this could be. It's '99 with about 90k miles on it and new engine at 40k miles. The car is 10 years old and has its fair share of wear but overall seems solid for the price.

Any input would be great