Is this from Corrupt media? Deleted but still wreaking havoc.

Hi there,

This is just about Final Cut X but I can't get a bite over there and am getting really desperate if anyone can help with suggestions on whatever else to try.

FCPx 10.0.9 on Mountain Lion iMac

Had a clip playing heck with a session, and replaced it in various ways. It was a music video from YouTube, supposedly the "official" HQ version) imported as optimized media. A while back it didn't seem to cause problems with playback or export. This time it was making monitor video go black when playback reached the first clip in the edit. Various other severe performance problems as well.

Just trying Digi Reb's Pro Maintenance corrupted file search for the first time, even though I pretty much knew which clip, but it did not confirm anything as corrupted. Also tried PMT's Quick Fixes, fixed main drive Disc permissions (should I try the video media drive too?,) used PMT and Onyx to trash preferences, did full Housecleaning function, all w/ trash empties and restarts. Checked video drive defrag status, not bad. Performance of Mac overall and FC improved over all, but still same problem.

Redownloaded the video from a Vimeo site, and this time converted to Final Cut optimized using iSkysoft Media Converter Deluxe, (which I've used for doing the same, using clips up to 20 min's as original media to great success, all much faster than optimizing upon import.) I relinked the clip to this new version, same problems.

I then eradicated every trace of the original corrupt files in or out of the session, first filling the timeline clips with empty gaps. Then imported the new download / conversion and replaced the gaps from scratch (easy enough, it only had 4 short clips.) This seemed to work fine in the edit, performance not great but much improved.)

First attempt at output, video all black. Tom W. suggested on another thread making an edit change (which sounded good because wheras the clip would make all Timeline video go black, making any change would bring video back, until hitting that same clip,) changing import settings (first output was Apple Device, SD res.) Second output, now "Audio + Video Master," H.264, was great until hitting the same first clip, then video to black just like in the session.

I'm guessing the Vimeo file may be the same as the YouTube file, w/ same problem, but

does optimizing video normally create a usable file, or reject a bad one?

More constructively, is it possible the original file left some corrupting element behind even though all file remnants have been trashed & emptied, and Browser clips deleted, and can it be found and corrected? I thought replacing the edits with gaps would be a good precaution but apparently not effective.

Finaly, if I download the video from other sites or other YouTube accounts, is there a way to test an original or converted file before inputing and going through the same thing?

Thanking you kindly,

Brian