I have:
shut down the phone and re-started it
Put flight mode on for at least a minute and then turned it off
Taken SIM out, cleaned it, checked for dirt
Each time the network comes on for anywhere between 1 minute and 10 minutes then goes off again
i haven't tried the SIM in another phone yet but my phone is working in all other respects.
Net work coverage says all ok.
Do I need a new SIM.