Network dropouts on Sierra driving me crackers

[rexone]

One of our iMacs is having serious connectivity problems.
Both ethernet & wifi will drop internet access. Happens daily, randomly & repeatedly.
The computer will still be visible on the LAN & can still access LAN stuff but it just drops internet.
This does not happen to our other iMac which is using the same network etc.
Sometimes forcing it to renew the DHCP lease fixes it, sometimes not.
The iMac with the problem is a Retina, 27", late 2015 model running 10.12.3.
This was never an issue before Sierra updates.

Any help appreciated as I don't even know where to begin to try & troubleshoot as it doesn't make any sense to me.

 

[casperes1996]

Any help appreciated as I don't even know where to begin to try & troubleshoot as it doesn't make any sense to me.

If it wasn't an issue prior to Sierra, I'd say a good starting point is installing El Capitan and seeing if it persists.

 

[rexone]

Thanks casperes1996. Logical suggestion but extremely impractical I'm afraid.
The time & problems involved with downgrading an OS are just simply enormous. Okay if you have heaps of spare time on your hands but not when it's one of the machines in our small business & I need to do the work to fix it (meaning I have to virtually shut the business down for a day or more to try & find a fix)
There has to be something that is causing this issue but I do not know how to track it down.
The fact that it remains in the LAN but drops internet connectivity is what I find weird. If it was a network config issue you'd expect to see it go dark altogether but it's not. It's just losing internet connectivity.