Sounds like a connection problem with either the socket contacts or the bulb contacts. Check that the socket contacts are clean and not bent or damaged. Likewise make sure the contacts on the bulb are okay. If everything looks good, then swap the the bulb from the Right Fog Lamp into the Left Fog Lamp, if you have the same problem then the socket may need replacement. If there is no problem then the left bulb needs replacement. You can also swap the left side bulb into the right side to see if the problem then occurs on the right side, which would make the bulb suspect.