King Arthur flour has a great starter.
www.kingarthur.com From my experience you may have let it set to long. It should take from 8-16 hr and is best to use just as it starts to deflat. If your using plastic it can harbour bacteria. Your recipe is good. Between 90-100 degrees is good. You might use a fraction of a Campden tablet which supress other bacteria and wild yeast. It can work even better than the salt. With the campden tablet you can make it with 3 tsp cornmeal ,1 tsp flour ,pinch of baking soda ,and a 1/2 cup scalded milk. Scalding the milk breaks a protien that inhibts bacteria growth. The key is to let it cool and let a film form on the top. Remove the film that the bad guy and continue with your recipe. Here 's a recipe from a baker in britian who's made SRB for 40 years.
In the early evening, set the pre-starter - Two cups of scalded milk, immediately after removing from heat,
Stir in two cups of corn meal, and
Three tablespoons of wheat gluten.
Cover the container loosely with plastic wrap or similar and place it in a space that can maintain a temperature between 95 and 105 degrees Fahrenheit. The temperature is important - ten degrees less and action slows dramatically.
First thing in the morning, make up the starter - To the pre-starter, stir in, One cup hot tap water (~125F),
One-and-a-half cup flour,
One-half teaspoon bicarbonate of soda.
Loosely cover the container and return it to warm place. In about two hours the slurry will be covered with bubbles or foam and will have increased volume by 10 or 15 percent. When it reaches this state;
Make up the dough, add to the starter - One tablespoon sugar,
One teaspoon salt,
Three tablespoons shortening (oil or solid), and
Flour enough to make a stiff dough (heat the flour till warm to the touch).
I fit the dough-hook, heat the bowl by rinsing with hot water, and add three cups of flour plus the starter. I let this slurry become somewhat uniform to then continue adding flour until the machine seems to groan (5 or 6 cups, perhaps). I don't have another rule of thumb for judging "enough" for the dough - probably five minutes total time stirring by the time the last of the flour has been added. The finished dough is somewhat sticky and seems tough.
Divide the dough in two, form loaves, and place in greased pans. Oil the surface, if you please. Put the pans into the heat box for about two hours when the dough will have risen to the pan edge. Bake in 350F oven for an hour or until nicely browned.