The contacts inside of the switches get built up with carbon and after a while it becomes excessive. If you are somewhat mechanically inclined you can take the switches apart and clean the contacts with some fine sandpaper or a green scouring pad.... it's not a difficult operation; I did my master window switch last week.
I had to clean the contacts on my master switch a couple years ago too... I'm sure it didn't need so much attention when the cars were newer; The fact that this occurs more frequently now that they are older has me stumped. At least it isn't a difficult project and once you've done it one time it's quick and easy thereafter.