There are threads here about similar problems and the solutions, including the spray, repairing worn surfaces, and replacing the whole switch assembly.
Some who have had similar problems have tried spraying electronic contact cleaner spray into the switches through the gaps around the buttons, and in some cases it seems to have worked. This is possible if the problem is dirty contacts, and if the spray finds it's way in to the contacts. It's worth a try; however, do it with the power off, and wait a good long time after for the solution to evaporate -- most of them are flammable and you don't want to turn the power on and try the switch when it's still in liquid form. Spray contact cleaners are available at electronic parts outlets and places like Radio Shack.
