A question about gravy

hello
I manage a small pub kitchen and we have been experimenting with gravy over the last 2 months. I hired a cook with "18 years" experience :crazy: with the idea that we could move from packaged gravy and soups to homemade gravies and soups. Its worked out somewhat decent so far for the soups but we have been having problems with the gravy.

Here is the recipe we are currently following:
16 cups beef broth
4 cups beef stock
14 oz margarine
14 oz flour
seasoned with salt and pepper.

I have a few questions for any chef out there. The gravy is tasty and thick the first day its made, but the day after its "watery" and has a strange taste to it. We have to add more roux to the gravy in order to get it thickened again and reseason to overcome the weird taste. We have been adding anywhere between 2 oz - 4 oz of flour and margarine the next day to re-thicken the gravy. I would like to know how to stop it from becoming watery and why it tastes strange the next day.
To help answer some questions ill break down the procedures we follow in making and handling the gravy.

First we make the roux, melt the butter over low heat and stir in the flour until they are mixed and let sit over low heat for 4 - 5 minutes. Then we add in the beef stock slowly while stirring.(Sometimes the stock and beef broth are made just before the roux and are still hot.) When the roux and stock are mixed we then add in the broth and bring the gravy to a boil where it thickens quite nicely and then season it. The gravy sits in a steam table for the entire days service. I have the temperature set at 70 Celsius inside the steam table. I hope this information will be helpful! Look forward to some replies :chef:

I would also like to add, I'm totally open to any helpful tips to help improve our gravy.(flavor and procedure) We use beef stock because beef drippings don't come by often enough in the kitchen as we maybe do a small roast once every two weeks.

thanks for the great replies. First, sometimes there is a small layer of oil around the edges, but not all the time. The only spices we are adding are salt and pepper, maybe i can try a different stock product. We don't use beef drippings because we never have enough around, as we make gravy sometimes once a day, but other days when business is slow its made once every two days. We also use margarine because we get it for cheap from our food supplier. I will look at the cost of no salt butter today at the store. I would go with my food supplier however his prices for dairy products are identical with the grocery store on the corner.

I have looked around these forums for other clues as to why we are experiencing problems with the gravy. One possibility is that its not being cooled properly or fast enough. It might also not be covered right in the fridge also, as i work days and some nights but am not around for close. Ill give these other ideas I've uncovered a shot and get back here with my results. :)
thanks for all the help so far :chef: