I am a CISCO novice so I really have no idea where to start with this. We have a Cisco 1801 router connected to an 8MB ADSL line. Up to about a month ago it was working fine. Internet access and download speeds were pretty good. However it has started playing up and while we can still generally access web pages (although sometimes it takes a couple of refreshes to get them rendered correctly), if we try to download any reasonable size of a file you can forget it. They will download so far then just stop (and this includes thigs like out antivirus definitions and anti-spam rules)
I had our ADSL line tested and it is fine. I also took our local network out of the equation by disconnecting our switch and connecting just one PC into the router switch ports but that made no difference either. Finally I reconnected our old Draytek router to the DSL line and everything worked perfectly hence I'm pretty sure something is up with the 1801.
I have attached the running config but any help on how to diagnose or resolve this would be greatly appreciated!