From the quick and dirty hack dept... Many 7xx/9xx Volvos experience the "Fan slows down when accelerating" issue. What is really happening is not that the fan is slowing down, but that the vacuum servos that control the flaps start leaking. Since there is little to no vacuum available while accelerating, these cars use a check-valve to ensure that the vacuum available to the climate control vacuum solenoids doesn't leak back out to the manifold during acceleration. Assuming that the one-way check valve (mounted on the firewall) checks out OK, the most likely culprit is a leaking vacuum servo. These are located under the dashboard on the drivers side (plus a hard to access one for recirc behind the glove box.) These are easy to test with a hand-held vacuum pump and gauge. If the servo doesn't hold vacuum, it's bad. The one that went bad on mine was the servo with the "double" diaphragm - the only one that has 2 vacuum hoses attached. This had a orange vacuum hose attached to the side and a blue vacuum hose on top. The diaphragm that the orange hose connected to was the one that was bad on mine. All I had to do was plug the hose with a rubber plug, and I'm a happy (and much cooler) camper in this heat. I did lose the "floor only" fan setting (can only get floor + defrost now,) but for me, that is much better than attempting the extremely labor-intensive operation to replace the vacuum servo. (Either via the "proper" way that involves removing basically the entire dash, or the "quick" way here: http://www.swedishbricks.net/700900FAQ/Heating-Vacuum%20Servo%20Replacement.htm#Replacing%20Vacuum%20Serhttp://www.swedishbricks.net/700900FAQ/Heating-Vacuum%20Servo%20Replacement.htm#Replacing%20Vacuum%20Servos%20in%20ACC-Equippe%20700/900vos%20in%20ACC-Equipped%20700/900 ) Here's a picture of this quick fix:
...then I just stuck the vacuum line back on the tab on the servo to keep it in place:
Not bad for free and about 30 minutes of testing/removing & replacing panels!