Several questions:

1. Have you seen any evidence of new staining?

2. Why is the cal hypo expensive? How are you buying it?

3. Do have access to a Sams Club? If so, buy their 24lb pack of 1# bags of 100% dichlor shock. Each bag will add about 2.5 ppm of chlorine, and about 2 ppm of stabilizer to your pool. Otherwise, order dichlor from Amazon:
Kem-Tek Dichlor 22 lbs
You need to start bringing your chlorine and stabilizer levels up. You can use the dichlor to raise both at the same time. I'd aim for 1 - 3 ppm levels. You can add the dichlor to your skimmer, but take the CuLator out before you do, and replace it after. I don't recommend buying dichlor locally, otherwise, at least until you are an EXPERT reader of chemical labels. The chlorinating pool chemicals sold at Walmart, Kmart, Costco, and most other local stores are diluted blends, sometimes with copper and other products with bad side-effects.