My screen goes black if I wake the device up and I had been browsing a web page in Dolphin when the device went into sleep mode...
I have to press the n button, the last web page browsed briefly appears before my homescreen comes up and I go back into Dolphin and it's fine again.
I've taken to going back to the homescreen if I am going to let the device snooze..
But your issue sounds much more annoying and intrusive than that.
A lot of times this can be due to a bad app. I don't know if you've downloaded many but you can start by uninstalling recently installed apps. Short of that, the best bet is to do a factory reset. Annoying though that may be, it could very well fix your issue. I had to do that once already because something wasn't working right. Can't remember what is was now but at least I didn't do the reset for nothing as it fixed the problem.