Sounds daft, but you havnt got a load of keys on the key ring???
If the lock is a bit worn, the weight of the keys after going over a bump could turn it off, or I have had my knee hit the keys and done the same thing.
Or as talking cars says, check all the earth and battery connectors, had a Volvo that would cut out, lose everything, then come back on. That was the power lead to the starter motor that had broken.
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