This is an easy one! There is either a small tear in the diaphram of your booster or one of the vacuum hoses is leaking. I'm not sure what the Volvo system looks like, but I would be willing to guess that there may be a "valve" somewhere in the system. This could also be sticking. (like a ball check type deal in the vacuum line , the big one) I know that a VW has a one way valve in it. If it gets crud in it , it will act this way. When the car "runs hard" it makes more vacuum , right? This may be just enough for it to work "a little", or sporatically. Check the "cheap" stuff first, cracks in the hoses, and the presence of a valve etc. It is definately a "vacuum" situation, also check to see if your vacuum pump is indeed working properly. Take the big hose off and put your finger/thumb whatever over the end of the hose while the car is running and see if it's making vacuum. ( before you blame the booster!) I had a friend who replaced a booster on his Rabbit when all he needed was to rebuild his vacuum pump. ( this was a $75.00 used vac. booster vs. a $12 rebuild kit for the pump!)

Let us know what you find! Tom