I had the same problem w/ 94 850. There are two small springs inside the turn signal switch that keeps the cancelling mechanism centered. When the crappy plastic boss breaks off one of them, the mechanism is only centered in one direction, the other direction then won't cancel.

I actually repaired it by drilling two small holes where the plastic boss broke off and carefully used a piece of 30ga wire, looped through the holes and the end of the spring. Twisted it tight, and, viola, it worked. I don't know how much a new switch is but if it's a normal Volvo, it's probably pretty expensive.