WMP is a lousy player. Don't trust it. Use VLC or Media Player Classic.
let me add my comment of another discussion:
My concerns are about a)distribution and general b)compatibility.
a) When videos are distributed most of the users will probably try to open the files with Windows Media Player. So it is not a good start to have fresh exported h.264 videos which won't run fine with the 'main' player on windows
b) I am wondering if the Problems with Windows Media Player may also be a sign of a general compatibility issue of the h.264 video itself
At least in case b) Adobe would have to look into this.
And in general there maybe a bug in the MS h.264 decoder.
I have a pretty decent system (see the PPBM4 results, Harm's Beast) and I can't get WMP to play a H.264 file anywhere decent. If most of your viewers will have less capable systems than I have, they will have even more problems. Even with Media Player Classic or VLC I need to increase processor priority for fluid playback. H.264 is just too large a burden on most systems. Maybe consider an alternative codec, like Flash.
Hardware Performance is not the reason for the jerky playback here.
Any h.264 videos run smooth on all test systems and with any player except these videos I just produced by myself with Premiere Pro CS4.
So there must be a bug in the h.264 decoder and / or a compatibility issue with the Adobe h.264 encoder.
Here comes the confirmation of Jim:
"I ran some more tests. WMP 12 on Windows 7 64 bit did have trouble playing both your MP4 export as well as my own. I just never noticed before because, like I said, I don't ever use WMP.
However, the F4V exports using the H.264 codec seem to play back just fine on all players."