Hang on I was a bit premature there. That's great if it fixes your port changing problem but as for your slow speeds... looks like Manx might be using traffic shaping. This will cause your speeds to drop. This article has no date, which is really helpful, but it suggests that this is what's happening
http://www.dyband.com/documents/CaseStudy-Mannet.pdf

There was an article too from 2004 that mentioned Manx using Sandvine, which is a package for p2p control. You might just want to give them a call & ask them whether they are employing traffic shaping & if so, whether it's just at certain times of day. Some ISPs only do this during their "peak" times.