PF S8 variable idle

[Heksi]

Have a look at the video. I am coasting on neutral, and as you can see, engine idle varies steadily from 600 to 900 rpm. It has never stalled and engine seems otherwise healthy. Strange thing is that it never does this when the car is stationary, only when it is moving. For example, if I coast and stop slowly to traffic lights, idle evens out as soon as I stop. Also, it does not do this all the time, but occasionally.

Any ideas?

I have a PF S8 AHC with manual transmission.