S80 MK1 Radiator fan turning but not fast enough
 

Hi there, I need to pull in the big guns for this.
It’s an S80 2006 MK1 runs like a little clock but for only one thing.
The Radiator fan isn’t turning fast enough!
So the fan runs turns on a slow speed but if you are climbing a hill, or it’s a hot day the temp will climb, I don’t let it go above 95 degrees. I turn the heater to hot and let it cool down.
Any ideas what it might be?
I’ve have a timing belt and water pump change along with the antifreeze but it was doing this before that.
It’s like the fan isn’t turning fast enough, like it’s on a low setting but the high setting doesn’t kick in if that makes sense.
Always grateful for any help
Thanks Glen