Ah, so you have connected using USB? Always a bad thing IME....it's crap software and the best way to connect is through a LAN card, which I guess you don't have hence the choice of USB.
The first thing to do is to switch off and switch on the broadband modem, leave off for about 30 seconds. When it is back on, leave it for a minute or two before using your computer again (restart if you want) and try again. Hopefully that will solve it simply.
Because it sometimes does and sometimes doesn't, it isn't likely to be your computer IMO anyway...Basically with your speed most webpages such be pretty much instant, only let down by your computer and by the connection of the website.
You could switch off and unplug the USB modem from your computer and uninstall (start/control panel/add or remove programs) whatever the software is called, I forget, something stupid
CorrectCOnnect? Then using the CD you go, reinstall the software and then replug in your modem and switch it on when you've done that, that should allow you to go into medic.
If the above doesn't fix it, I would probably phone up NTL and tell them the following:
"I have a problem with my 1.5 megabit internet connection. Sometimes when I try to load pages it works, but most the time it doesn't. I have been told that this could be a connection error and wondered if there was some way of you trying to help me sort it out please?"
They will of course say it is your computer lol - they might ask you to use correctconnect to work out the problem and it wouldn't look good if you couldn't get it to work.
Of course I assume you have tried adaware first Lora? You would be surprised how many slow/faulty internet connections are caused by adware/spywear that AdAware gets rid of....Try this first before all the above.
I am sorry that none of this is in any sort of order, I randomly wrote different bits at different times lol