I've had the same issue with the stock rom, and with the leaked sprint beta rom, although it doesn't happen very often. This was issue was talked about before... this thread doesn't have any solutions (except for soft-reseting and the switching the screen orientation back-and-forth thing).
http://forum.ppcgeeks.com/showthread.php?t=22759
As others have noted, this issue can occur on completely stock roms.