since we use direct show, the quality and functinality of video palyback strongly depends on you installed/configured codecs.Some features don't work correctly, like video playback.
so if you i.e. set MyVideos to use ffdshow for mpg2 videos then this could fail.
if you play the same file in WMP then it could work there, but since WMP is using IntelligentConnect its very likely that it uses a different codec, not ffdshow as you may configured in MediaPortal.
just an example.
generaly speaking your case is quite extreme.
i have seen PCs which had one or 2 of the issues you mentioned. but never seen one with all of them.