I have found a few cases where the PSOM was the cause of intermentant loss of VSS signal.

Photo 1 is a view of the speedometer cluster that we are all accustomed to seeing. Photo 2 is the PSOM attached to the rear of the speedometer cluster. At the bottom of the PSOM, there is a ribbon cable similar to what you would see in some printers. A PSOM that intermittently loses its signal can be traced to a partially dislodged and / or dirty ribbon cable connection. It is recommend using Isopropyl Alcohol and the brand name Q-Tips to clean the ribbon cable connection. In addition to ribbon cable concerns, there have been circuit board issues as well. You can sometimes see a burned spot on the circuit board if you examine it under a strong light and use a magnifying glass. If you ever remove a speedometer cluster and there are obvious signs that the PSOM has been contaminated with rain water, soda, coffee, etc., it is recommended to replace the PSOM assembly as opposed to cleaning as cleaning under such circumstances have not proven successful for long term repairs.