I found this thread looking for a fix for an MX 850 of mine. Cleaning the contacts didn't work. However, I found a suggestion on another site and it worked. I got a roll of metal covered duct tape, $6.99 at Home Depot, and used a paper hole punch to cut round pieces. I put a small amount of superglue on the back of the buttons that weren't working well, as the carbon had worn off. I gave the glue 30 seconds to start to drive and then put the metal pieces on the back of the button. So far, so good.

You could also try kitchen foil. There is also a kit sold on ebay:[Link: ebay.com]

You can find a video on youtube that shows how to install these buttons "covers." The idea looks good, but I didn't want to wait for an order to come in. I hope this helps someone.