I'm thinking its a routing cache of some form then. What ISP are you using these days? I'm thinking their green box thing (forget the name of the damn things all the time) at the end of the road or whatever has caught a dodgy cache now. When the problem is there, it's there a lot. You wouldn't get it intermittent to the point of it happening twice over hours compared to every few posts beforehand.
I've got it logged with the server management firm who will check it over properly tomorrow.
Though I've disabled a lot of features on here, we should be running awesomely quick now with no lag or double posting etc, and certainly not any 'database errors' (which aren't actually database errors but the software can't define issues well to be honest).