thanks wb, I have an Acer Aspire X3990 using an Intel Core i3-2120 CPU @ 3.30GHz, 64-bit OS, Windows 10 Home. I have confirmed that all drivers are up to date.
I'm not sure 82579V and 82579LM are both onboard NICs because when it reverts to 82579LM the info in Device Manager says something along the lines of 'is not currently connected' or 'not plugged in'. I'll check details more closely next time it happens.
I have noticed that the switch from 82579V to 82579LM happens after it comes back from sleep mode, and also sometimes when I turn on the computer.
I will get in touch with Acer to see if they have any suggestions and let you know if any solution is found.
cheers, Pete