I have had this problem before as well, so I am no help to you, but am also curious but never sought out the answer. Are you overlaying text onto video? My problem was I was creating lower 3rd’s for titles, and when I was bringing in the text I had it gaussian blur and fade in, while it was fading up it was like it was fading up from a black bg, instead of an opaque bg, like it should have been since it was an alpha channel, it must have something to do with the way we are outputting our alphas, sorry I can’t be of help, but it would be interesting to know what the problem is, for a short term solution I brought the text and lower 3rd bug in as seperate QT’s and did my opacity and blur settings through Final Cut, which isn’t the best result but worked for what I was going for.