When my CPS first started to fail, it caused what you are describing. On the expressway, the engine would stall and then re-start on its own due to the road speed continuing to turn the engine over. Your comment about falling on its face brings that to mind. When it did that, the tach would stay at 0 RPM until the next time I stopped and restarted the engine.
As the CPS degraded further it would stall right out, and sometimes the engine would not start. For my car, the issues seemed to get progressively worse over a 2 to 3 week period of time until I changed the CPS.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)