Dropping revs?????

[E46ACS]

Am having a strange problem.......

Traveling along at 80ish, cruising along, if I ease off the accelarater the revs drop quickly to around 1000rpm. When I press the accelrater, or change down a gear, the revs pick up to where they should be. This isn't just a dashboard issue or rev needle problem, as I can hear/feel the engine pick up revs, so it looks like the engine revs are dropping. The speed does start to drop slightly.

The revs don't drop instantly, but take a few seconds to drop, and doesn't happen every time I try, but probably 3 out 5 times.

There doesn't seem to be any issues elsewhere, or any impact on performance.

Any ideas?