Your problem is most likely related to a vacuum leak where one of your vacuum valves looses vacuum and opens a flap causing you to get hot air.
Try putting the fan on position 1 and turning off the radio and when you accelerate and then let off the glass, listen to see if you hear one of the vacuum valves.
Also check under the hood to see if there is a vacuum leak on one of the hoses that go thru the firewall on the passenger side for the air conditioning.There are some little valves in the hoses just before the hoses go thru the firewall that could be bad. They are about 3/4 inch long and white on one side and black on the other. They let the air move one way through the valve and not the other way so that when you accelerate and the vacuum drops, you will not loose your cool air.
But one thing for sure is that you should not loose the cool air on your V90 when you accelerate.
Jimo