Jeff,
I happened to be looking at the VSS on a clean schematic. The VSS module has 3 wires: +12V, ground, and a 0 to +5V pulse output. If there was an intermittent defect in the VSS module, or an intermittent ground wire, it could apply +12V to the ECM (and the speedo) on a pin that isn't supposed to see 12V. This could potentially cause the ECM to malfunction if it wasn't designed to tolerate this. Maybe this intermittent is excited by ~4000 RPM vibrations.
It's worth looking at. If not replacing it, maybe just pulling the connector or something and seeing if the malfunction stops.