Finally discovered the fix for the iplayer randomly stuttering and lagging all the time during movies and trailers etc. To fix this issue you will need to disable WiFi syncing on all your devices connected to iplayer. You can do this by selecting each device and in options untick "WiFi Syncing"
This immediately resolved the problem for me without even having to restart the iplayer client, no more stuttering and no more lag. Hope this helps.
