It could frankly be any number of things, but here is a list of what I find to be the most common stuff with mine.
Btw, over here, "wireless broadband" is called "mobile broadband" so I'll be calling it that, so I don't confuse myself
Firstly, I actually had that particular modem at one point. I found it to be slow and unreliable. Now I may have just had a dodgy one, my friend also had one, and had the same issues, so I doubt it;s a fluke but it's something to consider. I managed to convince my provider to replace it with a better one, (the ZTE MF112, which ironically is actually newer, despite the lower number)
I find that with any mobile broadband connection, especially at peak times (evenings/weekends) the connection has a habit of just occasionally crapping out. Basically the mobile "cell" (the antennae that the mobile connection is made through) can only handle so much, and if lots of people are using their phones/dongles (modems) then it overloads, and you have to restart the connection to get any sensible speed out of it. This is why I asked about torrenting, torrenting overloads the connection very quickly. Disconnecting/reconnecting fixes it in most cases. This is actually mainly due to contention: Contention - What it is, how it works. Or "Why is my Internet slow?"
Next thing to consider is latency (response times, or "ping". Mobile broadband has a terrible ping. I have honestly never seen anything short of 150ms even from a relatively local site like BBC. Slow response times, unfortunately mean a slow connection. At peak times I have seen this go up to 300ms or even more. This obviously adds on to the problem that I described above.
Of course it is possible that it's a modem problem. The first thing I would suggest is to uninstall the driver and reinstall it. First uninstall the connection software using Revo: Download Revo Uninstaller Freeware - Free and Full Download - Uninstall software, remove programs, solve uninstall problems
(the free version will be more than adequate). This will make sure that any left over bits will also be deleted. One thing to note about Revo is that if it finds leftover registry keys, the one's in bold are safe to delete, and the one's that aren't should be left.
Once it's been uninstalled, just check Device Manager, (Control Panel>Hardware and Sound>Device Manager) to check it's uninstalled the modem too, if it hasn't, right click it and then click "Uninstall" then unplug the modem, plug it back in and reinstall it.
I would also seriously recommend not using the connection manager. On my machine at least, it has a detrimental effect on my connection. Once you have connected once, there is no need to run it, the connection will be listed under the standard network connections list on the bottom right of the taskbar (the thing that looks like a computer with a big red X on it)
If none of that helps, I would suggest getting onto your provider. THey will probably run you through a load of pointless diagnostics, but they may also after all that replace the modem for something better, or at least be able to give you an idea of what's going on.