Decel rpm fluctuation

[AusA8]

Ok, first question on the forum for me so please be gentle.

I did a search and found a few g'box shift issues, but nothing along the lines of what I've got.

When going down hill I lift off throttle completely and allow the trans to control the engine braking (where fuel use goes to zero). Instead of getting a consistent brake effect, I get a fluctuation where the engine rpms go up and down about 200rpm and I can feel the car surging slightly as the rpms fluctuate in time with the engine braking coming starting and finishing.

I note the car is 2003 D3, has 120,000km's (75,000miles) and shes not had an ATF change due to dealer refusing to (I'm about to read the thread about ATF changes.)

Firstly, does this make sense and any ideas?

And, how do I know if the g'box map has been updated?