Why does the roux break down in my chicken velouté sauce? I am trying to make a veloute sauce which involves adding roux to boiling hot water mixed with chicken base (such as Tones or McCormick).  This sauce is then mixed with heavy cream at a later stage to create a sauce for potatoes au gratin.

After you have made the roux and cooked it to a blonde stage (3-5 minutes), the chef that taught me this instructed me to cool the roux to room temperature and then add the room temperature roux to the boiling liquid stock.  I let the stock boil for a few minutes after the addition of the roux and then reduce it to a softer boil (simmer).

The problem that I have encountered is that when simmering the sauce, the roux eventually breaks down releasing the fat (butter) into the sauce and thickening the "center" portion of the sauce.  The fat essentially separates from the flour due to extreme heat (simmering).  What causes this, and is there any way around it?  Most veloute/supreme sauces call for a 30-50 minute simmering of the sauce to reduce any starchy taste and grainy texture.. but if the roux breaks down in the sauce before this how is this even possible?

If I don't leave the sauce at a boil initially for at least 3 minutes and then don't simmer for at least another 2-3 minutes, whatever remains from the lump of roux that was added settles to the bottom of the pan or forms lumps which eventually settle out.  The sauce also has a "grainy" texture and seems to have too much of a starchy or floury taste.

Any advice on this would be appreciated.  Thanks.
Per SAJ14SAJ's comment, the suspension in a traditional veloute relies on the gelatin to keep everything in place. Stocks are naturally rich in gelatin, due to the bones and connective tissue used to make them. Using a base to make the sauce is probably not providing the gelatin necessary. Other rich liquids frequently do separate when simmered for a long time (think curries and gumbo), so it's to be expected that your emulsion will break down eventually. I think your solution is to either use a real stock instead of the base, or perhaps fortify your liquids with a bit of gelatin before adding the roux (or barring those, cook only until your starch is gelatinized).