ScanGauge-II sometimes reads zero mph
 

I've been having strange readings from my ScanGauge-II. I'd be cruising along & suddenly the "tank average" gas mileage would start dropping for no good reason. At fill-up, it's "fuel used" is still very accurate, as it is while driving. I finally discovered that the mph reading would suddenly drop to zero - or maybe 1 or 2 - - - or 4 mph. But it's erratic - sometimes it will be fine (compared to the analog speedometer, which still seems to work fine).

So I wonder if the problem is in the ScanGauge or in the vehicle's speed as reported on the OBDII? This is my '99 Suzuki Swift 4-cylinder, 5-speed manual. Anyone have any theories? If it is the vehicle's speed reporting, does anyone know what would be involved in fixing/replacing the culpret parts? - and cost range?

Thanx

Does anyone know what causes this?
 

It's very frustrating having my mileage display "freak out" on me & throw my instantaneous and average mpg get skewed by this.

Does anyone have any ideas if this is a scangauge issue or a vehicle speed display issue? Is there a way I can test it? The analog speedometer on the dash always seems to be fine.

Nobody?