I have a hunch there's a new relay purchase (or chassis modification) in your future, so this advice is given in the spirit of postponing the inevitable...
Since you've already worked the contacts with a jeweler's file, chances are the surfaces are a little rough at the microscopic level and may have developed some new pitting. If you have access to some De-Oxit contact conditioner (expensive and hard to find, but it works!) wet the surfaces, let it soak for an hour or more, then work the contacts with the finest grit crocus cloth you can find at your local auto body / painters supply outfit. Cut the cloth into thin strips, use light pressure, flip the paper frequently (both cutting side and direction), and take your time until the surfaces look polished. Use bond paper as mentioned previously for the last few passes, and I prefer to work 'wet' when polishing surfaces. In other words, wet the paper then drag it through.
A few weeks ago I discovered that Lowe's carries CRC 2-26 "Lubricant spray with electrical properties" which is the poor-boy version of De-Oxit. It's intended to clean, lubricate, and penetrate contact surfaces to increase their electrical conductivity and reliability.
That sounds like what you're looking for...
For more info, visit: http://www.crcindustries.com/productinfo.asp?productno=02004