My Omnia used to do the same thing- it always happened when I had too many programs running on the handset. I would usually have to quit all the programs, then turn it off, and back on. It was a pain...
I was running version 5 on it (not sure about the rest of the numbers tacked on to the end of it, because my phone was water damages so I can't check).
I now have the Samsung Icon Hd, and it works much better on that.