I was driving to work the other morning (at the ungodly hour of 1:00am) when I hit a bump along I-25. It's a particularly rough section just past a bridge. When I hit the bump, both the tach and the speedo dropped to 0 and then popped back to normal after a second or two. Haven't seen it since, even driving over the same section. Probably a loose connector or wire somewhere.
Did your tach quit, too?