Originally Posted by Charken
One other thing to note which I mentioned above, is that if I move the cable that links the 8 port switch to the 5 port switch to the Virgin Super Hub or vice-a-versa the speed goes up to around 30MB/s for a limited time. I don't know how long this will last as I haven't tested it properly, but will work for some days.
I don't know what happens to drop the speed after some time.
This tells me that your 8 port switch is having erratic behavior. Only when you remove this switch does the problem go away.
Try replacing with a different switch if you have one, and see if that fixes your problem.
Or, conversely, you might be having a problem with the 5 port switch, since you still have the problem a few days later. I would bet money on one of those switches being the problem.