I rebuilt my server (from scratch as I also changed disk drive layout for EFI etc) as part of my update and thus my system was going to start using NetworkManager without still having ifcfg files for my devices etc.
I had an error in my ifcfg configuration which NetworkManager would not allow.
dhcp settings for my clients was fine etc.
For my server I had a fixed IP addresses and my server was a .1 address. I had my gateway for this connection as .0 address.
The reason is that the server is also acting as the firewall and using a second card as its connection to the outside world and that internal network card really did not talk to a gateway etc.

NetworkManger would start up my card (network connection) and would then try and connect to the .0 machine which did not exist.
After a minute or two it would decide it failed and shutdown the card.
If I restarted it then all was OK for a minute or two and then it would shutdown again.

All was clear to me when I used:
journalctl -b -u NetworkManager
The log told me exactly what was happening. Change the gateway and presto all good again.

This may have been mentioned as a problem elsewhere. Clearly the real error is mine in that the gateway error should have been shown earlier and NetworkManager in fact is doing the job correctly. Hope this hint helps someone else if they experience this when changing over.