Why phone sometimes (often) goes crazy and constantly lightens up its locked screen? In general, from time to time, my phone (Galaxy SII) lightens up its locked screen for 4-5 seconds.

I thought: the reason behind can be to show if some messages or calls are awaiting to be noticed (but, with screen locked, it is pointless anyway, because nothing is shown besides symbol entry screen).

But sometimes it lightens up the screen like 3 times in a minute, over 15 minutes.

Why it does that?
Try uninstalling some applications. The same thing happened to me and I fixed it by uninstalling my most recently installed app.