As I mentioned in ratman's thread, I have experienced a similar problem with NNTP downloads in the past couple of weeks.
Using "Process Explorer" I was able to identify that the Emsisoft Network Filter (in the network adapter properties) was causing the "interrupts" process to take > 50% of the CPU.
This was confirmed by disabling the Emsisoft Network Filter: when disabled everything worked normally.
My solution was to delete - then reinstall - Internet Security, something that has unfortunately not worked for you.
OTOH it may be worth testing with "Process Explorer" as a means of identifying the problem.