May be a fault with the main relay (engine management).

Used to be a common issue when I worked for Seat (more or less the same thing).

If the main relay isn't functioning, won't power up the ecu and the pump won't prime. Unfortunately I can't remember which number the relay is!

Next time it does it, check to see if you have a 5v feed to one of the terminals on the air mass meter. No 5v feed means the ecu isn't being turned on so good chance it'll be the relay.