black artefacts or Hang on .mp4 video using Flash

Hello

im new on jwplayer - at the moment i try to integrate it to my Website - so i want to serve both Wrolds (IOS, HTML5 and Flahs i decided to encode the VIDs do .mp4 using FFMPEG.

On Chrome or IOS Devices (IPAD, Iphoine) the Playback is in realy good quality and runs smooth with low cpu usage.

I also tryed it on the same Machine using IE8 and Firefox (both using FLASH Player) now the same VID is realy bad - on fast movements on the VID there are al lot of "black" artefacts on the vid on a other Machine (a old single core Notebook) also on chrome the vid runs smooth with IE or Firefox the browser need 96% cpu and the vid hang from time to time.

I also tryed to convert it to .flv with this format even the "snlow old laptop" can playback it - but the Quality is realy bad and no IOS support.

What can i do - or what make i wrong ?

I also tryed wmode="direct" no changes at all...

Best regards
AXEL