iTunes won't play videos without stuttering (PC x64 11.1.4.62)

I have been having this problem for as long as I can remember, and I've finally decided to fix it. Whenever my library shuffles to a video, it takes about 5 seconds to initialize, then plays jerkily, with the audio/video out of sync and low framerates. I don't have QuickTime installed because iTunes couldn't load the Store when it was installed, so I'd like a fix other than "install QuickTime." (I've already tried that to no avail.) I have tried using both my Intel HD Graphics 4600 and my NVIDIA GeForce GT 740 with iTunes and I get the same result-- this isn't a hardware issue. Does anyone know why a brand-new, high-performance computer can't play a minute long standard definition video? Any suggestions would be appreciated.

Thanks,

E-swizz