Black thing on top

I just recently got the iPhone 3GS and it does some weird thing every now and then, you know when you call someone and if you go into the menu during the call it makes a space (green, with information on the call) for you to tap on right? Well, my problem is that even when I don't use that feature, suddenly it decides to leave the space for that green bar and it is just a black space which has nothing on it and makes everything on the screen smaller because of that black bar on top.
I have turned it of and on several times and when I do it goes into let's call it "fine" mode, but sometimes it will just put it back (the black thing).
Anyone knows how to fix this?
Thanks