iMac frequently losing authorisations

A couple of years ago my iMac died completely - just suddenly refused to boot at all. I had it fixed under warranty, though I had to send it straight back the first time because it wouldn't boot when i got it home, and ever since then, it quite often loses authorisations for various bits of software.

It depends on the authorisation method how serious this is - for iLok stuff like pro Tools, I just get a dialogue box saying to authorise, then it's OK, but for PACE stuff like MaxMSP, I have to reboot (actually, shut down then reboot - restart doesn't seem to work).

This is not a huge deal (just like going back to Classic, when you had to reboot every time you installed anything!), but it's annoying when I'm in a hurry and i do worry that one day it's going become more serious.

It's not clear what was replaced under the warranty work, but one clue I've had about the possible cause of the problem is that one piece of software I've had problems with (I forget which) claims that there is no ethernet controller present when the problem occurs.

Can anyone cast any light on this?

3.06GHz Intel Core i3 iMac, running OS10.9.5 (was running 10.6 when the problem first appeared)