Car idling at 1000rpm

I have 2013 b7 passat 1.6 tdi.

My problem is, sometimes the engine will idle at just under 1000rpm instead of 750rpm.

This happens every so often, when I park up for a while, I'll go out, start engine and will idle normally again.

Will drive like this for 3,4 or 5 days and then starts playing up again.

So any ideas would be great because I'm at a loss at the moment to what the problem might be