Rose,
I have this same problem. It's very random, and it's been happening for years before CPAP.
With my latest cold/lung infection, I started taking Mucinex at night before bed to try to combat the coughing. It's helped me tremendously. You might give that a whirl. I haven't had a bad coughing fit that made me take off my mask since I started doing that last week. I was mixing it with a dose of Robitussin DM for the first few days. Now I'm just taking the Mucinex by itself.
I believe my coughing is more related to post-nasal drip and asthma/colds/allergies than GERD, but you'll hear all sorts of ideas here.
BTW, the Simply Saline is NOT a nasal wash, and will NOT work like a Neti Pot. It is for moisturizing only. Think of it as the difference between eye drops like Visine, and flushing your eyes out with running water.
The Neti Pot is cheap and well worth trying. I prefer the squeeze bottle version. I get it at Walgreen's for about $9 for the bottle and 50 packets of solution. It's a little weird to learn how to use it, but after you figure it out, it's a no-brainer. Takes me about 30 seconds to use.
Cheers,
Babs