Running cold on motorway, not thermostat :/

[porka]

Thanks for the replies guys.

I took the viscous fan out all together last night and went for a quick drive. No difference. Something happens at 2200 rpm and the temp. goes down, regardles of gear. Below that, it is normal.

I have found more references to this problem with 5 series owners:

BMWLand: Where Power Meets Control - UK BMW - 530D E61 - what temperature is normal?

The only trouble is that the fix is the EGR thermostat and I don't actually have one on mine. There seem to be two versions according to realoem.

Another possibility are cooling flaps which open and close electronically. Can't find anthing on realoem for my car but will go and have good look soon.