in these situations i'd recommend nearest nokia care, there maybe youtube videos about how to disassemble and re-assemble your phone but anything goes wrong will be like KABOOM wtf did i do (x5 times)
well, they replaced my digitizer and now all is OK.
@ AgogData with a little bit of technical knowledge and patience it's very easy actually. I took me 30 minutes to replace a broken screen on my other N900. But this one was under warranty so why bother.
well, it's not. And I believe that digitizer has nothing to do with it. I think that some electric contact isn't good because each time the problem with screen occurs after grabbing the phone by the top left corner everything works OK.
edit: not exactly a comfort for the author of the thread, but I got a new motherboard this time. And a new start of two year warranty.