Vauxhall Astra 06 plate

Hi, help please.
Any car mechanics out there able to give some advice please?
06 Astra
- On occasions car does not start - from cold or warm
- This does not happen all the time, very intermittent
- Can happen after a long journey or short
- 1st issue was with sim codes - car was re coded
- Happened again, new sim and keys re coded
- Vauxhall have checked:
starter motor, all fine
checked battery, all fine
checked alternator, all fine
Keys checked and told ok (although now telling me maybe i should pay for a new one and see if that sorts issue? £150 per key i dont think so, as dont beleive that to be problem. This all started when car first died and error codes said it was to do with sim)
- Cant check immobiliser or transponder as these are not things that can be checked apparently, and no lights or error codes showing
- When leave with Vauxhall garage it starts every time!!
- No error codes coming up on car when plugged in
HELP PLEASE as tired of being told if no error codes come up or lights on dash then we cant repair.