The stop does need agitation - the stop is to wash the developer off so it doesn't develop the film any further, it's a tres important stage. I make a lot of water changes in the stop stage, first one after about 10 seconds, second 15 seconds, then extend them to 30 seconds for another couple of minutes, agitating all the time. The first couple will contain far more developer than the later stages, so I like to throw it away as quickly as possible to remove the developer from the mix.
If it's going cloudy with streaks, just sounds like you're not giving it enough of a whizz round in the photo-flo.
When you say you gave it 11 mins 30s, was that 11 mins 30s in the developer, then add the stop, or are you getting it into water at the 11 mins 30 mark? You need it in the water on the mark, as it will still be coated in developer even when it's been removed from the solution.
All the stages need agitation really, if you can be arsed, agitating constantly throughout the process isn't going to do any harm.
You could always knock a little more time off the development if you feel it's over-doing it, but it's possible the scanner is over-exposing too, I had that problem. You could try loading into photoshop and using the curves to select the unexposed edge of the frame as the black point and see if that improves it at all? Or you could stop down your images, although it'd be better to get a reasonable neg. before you try that.
The image above doesn't look far off really, looks like it was a fairly bright day - did you meter for the highlights?