I've got a 5 year old Focus 1.6TDCI Sport and just before Xmas I had a new seal put on the turbo as the old one frayed away and a new radiator as apparently my old one has corroded away and coolant was ''***** everywhere''. Now since I've had this done, the heater only seems to work when I'm going faster, around 60-70mph but as soon as i drop below that it starts blowing cool again. 

To add to the torment, my engine temperature was fluctuating between 75-90 degrees for a few days and now is doing the complete opposite. I've had the Engine Temperature Warning flash at me  times today only for it to drop back to the middle [90] again after about 15 seconds and stays there for a while. I have no idea why it would be doing this, it didn't happen before I had the radiator done so not sure if it may be linked in some way? 

Another thing i've noticed is there seems to be a little whining noise when I press the accelerator in so far, at cruising it doesn't really happen, only when I'm trying to pull away and tends to be between gears 3-5, although on my journey home today, i was doing it even when in neutral.

Could someone shed any light on this please? I've read in places it may be a dodgy thermostat but I'm conscious of the whining noise too.

Thanks in advance

Air lock in the cooling system.  Just needs to be ran hot with the coolant cap off for a while to purge the air out.

Failed thermostat would only over heat or overcool, not both.

Thanksm may be a silly question but do i get it hot first then take the cap off or take it off when cold and get the engine hot?