My iPhone is using iOS 4.1 with modem firmware version 1.59.00 and doesn't stutter when doing what you've listed. Maybe the current modem firmware bug talked about in the Apple support forums has something to do with this.
Is there anyone with 4.1 and 1.59.00 having this issue? I realize this isn't the "Hacks" sub-forum but it may help get to the bottom of this issue which some are experiencing.
Mine is 03.10.01
Hmm. Strange.
Why would the modem firmware make a difference? Why does it happen after landscape mode?
By the way... The iPod touch also does this an that doesn't even have a modem firmware.