I have the exact same problem. It also started about a week ago. The GPS signal seems fine. The VSS wire is hooked up and reading correctly. The speed and heading reported on the instrument screen is correct as well (and the LA map rotates correctly with my heading), so it doesn't seem like a completely hardware issue. I reset the unit once and it fixed it for a short while, but the started having the problem again. A subsequent reset didn't fix it.