Hi, I have the same time-flash issue. While typing, the black time screen will flash at random times when typing. It always happens when my phone gets to a lower data connectivity type, as mentioned above, it happens when the phone has Edge connection and not 3G or HSDPA connection.
No settings changes have fixed this. Funny thing is, I had this issue with the samsung galaxy S1, and have the same issue with my S4. My brother that also has an S4 dont seem to have this issue at all, but he's with a different service.
So from experience, on all galaxy models I had/have (and in different countries), the same problem occurs when I'm on Edge connection. I wish I could find a proper fix for this or that Samsung could have done these tests.
Another suspicion; if it's because of bad reception (down to Edge) why is it only on certain service providers? Does these in any way effect a refresh/resetting in function or code transmission? Maybe that doesn't make sence, but maybe it does make more sence than seeing the time refresh 10 times while typing a short message.