Why does my red wine reduction go 'grainy'? After frying some meat, say beef or lamb, sometimes I want to make a red wine reduction. 

After lifting the meat out of the pan I turn the heat down to low and add the wine. The wine tends to bubble furiously and reduces quickly. I quickly try to scrape the meat remnants as it goes.

Trouble is, the sauce ends up looking grainy with dark spots in the sauce, and it tastes bitter.

The wine is always wine I wouldn't mind drinking.
I have to wonder if you are using a wooden utensil to 'scrape the meat remnants'.  If so, stop using wood.  The wood will char and leave bits in your sauce, creating the bitterness.  Try slowing down the process a bit and using less heat, at least initially as you deglaze the pan. The graininess could be charred bits of food which are burned before they are loosened from the pan surface; this could account for the black spots too.

Cooking is love, and love doesn't hurry.

:)