Drop Top 06

I have an 06 gt with the stock 5 speed. Has anyone else experienced problems downshifting from 5th to 4th? I've had it happen to me twice on the highway. I've only got 6K miles on it, and haven't had problems with any other gears. I know easiest thing is to take it back to the dealer and have them rake me across the coals to say there is nothing wrong. One of my coworkers said something about the synchro's or something. Any ideas?