I once had a problem with similar symptoms (but before setting off...) and it turned out to be the connector on the AFM was a bit corroded and dry. A bit of contact cleaner/lubricant and it was fine.
If it's not that I think I'd be looking for a camshaft or crankshaft sensor that is going open circuit when it gets a bit warm but difficult to diagnose until it fails completely.
...peter
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