The nitrates that you have won't kill coral, and the rest of your water parameters are OK, with one exception. Presuming you're using an accurate test like the Hanna Checker 736, your phosphates are very, very low. It is absolutely true that folks can and do keep SPS in very low nutrient conditions, but these tend to be highly skilled SPS keepers, and they tend to make up for very low phosphate concentrations by feeding coral foods heavily.
A lot of us, including me, keep phosphates around 100 - 300 ppb. That's low enough to keep algal growth in check, but not so low that it will shock SPS corals purchased from a high nutrient system. That, along with the alkalinity swings from dosing once per week, is likely to be the primary cause of your losses. You might want to test the phosphate, nitrate and alkalinity of the water from the LFS (or 'net retailer) that you're purchasing your corals from. If the corals have been kept in a high dissolved nutrient environment (NO3>10 ppm, PO4>300 ppb), putting them in an environment where the phosphate is very, very low often doesn't work out well - they have to be acclimated to low phosphate levels. Ditto if the dealer's tanks run a low alkalinity and you suddenly put them in a high alk tank.
Your nitrate levels are obviously not an issue (i.e., not too low), and the alkalinity that you state in your first post isn't drastically high, but there's an almost guarantee that the alkalinity swings are too much for SPS. As a point of reference, I currently hand-dose 2 tanks with SPS (another 2 are on dosers). These two hand-dosed tanks get a carefully measured dose of alkalinity and calcium solution every night.
