After restarting the machine, it looks like it's averaging out at a much more sensible 40 MB. Sorry for the bother. Great app by the way.
I've looked so long for something akin to NetGenius. Great UI, good features, great design decisions. And now, excellent performance profile. The only thing I'd change is add more firewall features. I think I'll stick to simplewall for now in that aspect.
If it's any clue to as to what went wrong before, there were many processes accessing the network and Windows was also updating itself I believe, including an update to .NET Framework. Though the NMCore.exe doesn't seem like .NET from what I can tell. Curious restarting the service didn't solve it.
That could be the case if hundreds of connections are open at the same time, which is possible when running a Windows update. Once those connections are closed, the memory usage should drop to tens of MB, just as you described.