HELP NEEDED

[mills367]

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

[TomsFocus]

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.

[mills367]

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?