Doesn't really matter which spa or brand it is, well some does. But it sounds like a high sanitizer demand, in which case you need to up the sanitizer. I don't believe Sundance's sun purity will work with Bromine. I know Jacuzzi's doesn't and it's pretty much the same thing.
If you have a lot of people in the spa at any given time in the day/night, you might move to a bromine floater. Ask your dealer about it, and see if he can help set you up. Add some granular bromine then set the feeder to #2 or 3 on the side and throw it in. Shock once a week with a non chlorinated shock, watch PH/Alk and go from there. Foam can be from a number of things. How is the calcium level?