Gotcha. That would be the i9100. I moved you to the Samsung Galaxy S2 (International) forum.
Now, on to your problem.
Have you installed anything recently or has anything happened to your phone? That behavior could be a variety of things. What it may be is the phone running out of resources because something has gone rogue and won't release ram or the CPU.
I would first try a soft reset. Turn your phone off and remove the battery for a minute or two. Replace and reboot and see if the problems are gone. If it works for a while and then the problem comes back, you might want to install Watchdog Lite
and monitor the phone's resources.
If that doesn't find anything or solve the problem, you should consider a factory reset. This puts your phone back to an out-of-the-box condition and clears all user data, so make sure you have ALL
important data backed up. If you're rooted you can use Titanium Backup
to backup the entire phone. Frankly I usually just backup my messages using SMS Backup
, copy off my media files and any other personal data and reinstall my apps, but it's strictly personal preference.
Here's how to reset your phone.
Samsung Galaxy S2 - Hard reset (ITVraag.nl) - YouTube
If you do that and it is still not working properly, then you most likely have hardware problems and only a warranty replacement will fix it. You could always pick up a cheap Pay-as-you-go phone and use your O2 sim in it until your SGS2 is returned.