Why the hell does my Internet connection randomly disconnect?

I got a broadband called "Telia Mobile Broadband" a few weeks ago, which was obviously supposed to be better than "Telia Mobile Connect" (which ALWAYS disconnected and refused to login at all about 60% of the time, at least on my computer); and sure, it seems much better, but this one also tends to disconnect when I least expect it.
I can be surfing the Internet completely normal, and then the broadband suddenly logs out and I have to login again.
Why is this?
This is absolutely stupid and I see no sense in it at all.

A lot more information will be necessary to diagnose the issue; First of all, what technology does your ISP use? For example, is it cable internet, DSL, WiMax, or dial-up? What is the manufacturer of your modem? What lights exist on it, which ones are on, which ones are off, and which ones flash?

Looked it up, it's closer to WiMax than anything else; it looks like you're using a cellular phone as a modem. That could be your whole problem right there. There's RF interference every-bloody-where and not a whole lot you can do about it. It may be a coverage issue on Telia's end. You'd have to call them up and see if they have any suggestions. I'm not going to look this up myself, long distance rates to Sweden would be prohibitively expensive from my location here in Canada.

More questions: Does anything pop up on your screen when you disconnect, or do you just get a 404 error when you click a link? If something does pop up, what does it say? Are you connected directly to your modem, or are you using a router? If you're using a router, do you connect to it via an ethernet cable, or via wireless? If you connect directly to you modem, are you using an ethernet cable, or USB? What operating system are you running? What IP address is being assigned to your computer? (NB: The staff may be able to see an IP address on the OP's post--this is not necessarily the IP needed to troubleshoot the problem). Is your IP address being statically configured, or assigned by DHCP?

If you're on DSL, do you use a PPPoE icon on your computer, or is your login information plugged into the firmware of your router or modem?
According to the article I read, Telia Mobile Broadband doesn't require a username or password; Perhaps it authenticates via your phone? I'm not really familiar with the technology, I can't say for sure. One way or another, you should not have a broadband connection icon on your computer, that's for PPP (Dialup) or PPPoE (DSL).

tl;dr version: using cell phone towers as a carrier for internet traffic is a terrible, terrible idea that should die in a fire. The article I read promoting it was directed at people from outside of Sweden traveling inside. Since your location says you live in Sweden, you'd be better off getting a more reliable type of connection such as cable or DSL. I don't know if Telia has such a thing because their website is in Swedish.

I'm sure telia has such a thing, maybe he's living in a rural area, or maybe he's been a victim of terrible advertising.
"Mobile broadband" like what it seems like he's using uses a phone-like device (except there's no display or buttons on it or anything) that you plug into an USB port, and you usually install some custom software to connect to the ISP.

Do you have good cell coverage where you use your computer? Can you call people with your cell phone without problems?

We have the same thing here in Canada, we call it the Rocket Stick and it's a scam. You get terrible speeds, unreliable connection, and a ridiculously high bill in the mail for bandwidth. DO NOT WANT.

There's a reason that Cable and DSL are so popular.

I was actually scammed by a similar deal here. Lucky for me I didn't commit to the 24 month contract so I cancelled it and got onto real DSL as soon as I had the time to do so.

I won't say what company it was with out of respect to their good work in other areas, but I will punch Richard Branson in the face if I ever meet him in person. ^^

Broadband over a mobile network is an abomination. The signal strength is too volatile, causing horrible attenuation and the end user sees that frequent disconnections, peaky connection rates and especially terrible performance in poor weather. High-speed internet is too sensitive a technology to be implemented effectively over mobile networks.

Having said that, you can maybe improve performance by moving your computer (or the receiver) to the part of your house which gets the best mobile phone coverage. And if that fails there might be a high-gain aerial you can attach, but that may cost a bit of money.