puppetlady,
How much magnesium are you taking? It might be more magnesium you need than potassium. I too have fibro and had the cramping, but what my doc told me was I was magnesium deficient. I had to take 2200mg/day of magnesium for about 6 weeks before my deficiency began to come around. Today I still take a lot of magnesium, 600 to 1200 mg a day. I take it according to what my bowels will tolerate. That's actually a good way to figure it out. My doc told me to take it until it gave me the 'squirts' (her words, not mine <g>) and then back off about 200mg. That's how we figured out my initial dose of 2200mg. Magnesium is very safe to take even in high doses, so give it a try. (I like the Nature's Way elemental magnesium the best, and I've tried a lot of different brands)