How To Choose Codecs In A Player

I am having trouble on a couple of PCs, in that the playback on most players (VLC, Media Player Classic, WMC) is jittery in some cases and fails in others.The jitter occurs when I go from a 108050p source to any sized output but at 25fps (i or p) output. The difference between a smooth play and a jittery play is that if you freeze, the smooth play is blurred, twice as much as it was at 50p, as if some sort of interpolation was being done, while the jittery play when frozen presents a still screen with several distinct ghosts of any moving bit. This is what leads to the flickery appearance.I was told that this was an interlace problem, but I thought interlacing caused horizontal lines on the screen, not ghosting.

I have edited / rendered video in various programmes. I have used AVS4U, Vegas, Adobe Elements and several others. The results are the same.I have tried many formats and codecs, as well as trying interlaced, Upper and Lower FF, as well as Progressive.However, at present, WMPlayer is playing stuff nice and smoothly for me, while none of the others are. All are jittery.In addition to this, WMC refuses to display vob files at all. IT just shows a blank screen, with sound, but only the progress bar at the bottom, ticking off the seconds, with no actual video at all.WMPlayer seems to be the one player that does not allow manipulation of codecs, and it just seems to be working....now.how do I choose what codecs are used by a player to play a file, or how does it choose?