For all of you trying to optimize the performance of their systems for a sale night - please do not try too hard, because the sluggish behavior and the errors are not at our end, they are server side.
I could explain that stuff for hours (I am an IT architect), but basically it comes down to the server being based on a solution (Joomla and virtuemart) that is very common and free, but simply not built for a sale night that can best be described as a distributed attack.
There is only so much that Robert can do with the server without redesigning the whole webshop. He upgraded the server twice already, but without special optimization, that won't do much.
The tens of thousands of requests that hit the server in a sale just can not be handled fast enough and the poor developer can not test or develop for that situation, because it is almost impossible to simulate thousands or even hundreds of antsy REOnuts.
Robert could build a solution with a real three or four tier architecture (balancer, app server, static server, backend), but it would only be used in a sale night, and how often are those? The servers (and the cost for them) would be wasted most of the time. And IT architecture is not Robert's job, either, I prefer him churning out mods =)
So, to conclude - we will have evil sale nights in the future, and we can't do much about it.