Mobile Network goes off often in my Galaxy s2
I also have a SGS2, and have had the same problem 2-3 times only. I turn on flight mode, and back. This solves the problem for me since it doesnt happen so ofthen.

if you still have the problem after this, I would re install Android to the newest version. Either do it yourself, or go to a phone shop and pay 10$ or so for them to reinstall your phone.

If this solves your problem, it was a cheap fix. If it didn't work, you can go back to the phone shop where you bought the phone and make them fix it in guarantee.

If you want to try to download and install an awesome rom for your phone, you should read this: http://www.cyanogenmod.com/devices