Resulting video is bad always on the same place I'm using Sony Vegas pro 10 and trying to render a small video for YouTube (3 min).
Resulting video is always slightly flushed, frozen, interrupted... on the same place - on about 1:18 till 1:36.  

Original video is ok. I tried to render just this small problematic part - the same result. Something is wrong on that part. Playing in Vegas - no problem.

What could be the reason, pls.

Maybe important, on that place the audio track is the most loud.

Rendering another videos is ok.
Two possibilities that come to mind are that it could be a dropped frame that isn't that noticeable until an H.264 compression is applied or it could be that you are using an insufficient bit rate and that particular part of the video has enough going on that it couldn't compress well.  

I would check the time code around the problem to make sure all the frames are there.  I would also try increasing the bit rate used and try VBR 2 pass and see if either of those fix it.