Clio random starting problem?

Clio16V Expression 04 auto.
Have random starting problem can go for weeks with no issues.
When fault occurs the immobiliser light does not go out and nothing happens when the key is turn (odd times engine turns for a sec but does not fire). Keep turning the ignition on and off will eventually start can take sec or minutes, have tried spare key with no success.
Had it on diagnostics at 3 garages one been a main dealer but no fault shows up.