Q: why do my music videos (mp4) play on my iphone 4 but not my ipad 3?

i have quite a few mp4 music videos in my itunes library, they will transfer to my iphone 4 and play fine, but the will transfer to my ipad 3 but only play the audio portion,

this is not related to the above question, but thought i'd ask while here

also for some reason lately they all play slow on my window3 computer and never used to. everything is defragged and on sata drives, i have even tried moving to a different drive but no luck, seems like one of the upgrades to itunes was when it started.

thanxs in advance