I have constipation and have to take quite a bit magnesium aspartate to control it. I've tried other magnesium, but get gut cramps and/or diarrhea. I have read that one shouldn't take too much aspartate. Could that be a problem?

Use magnesium oil instead. Health and Wisdom and Ancient Minerals are good brands. Either spray it on your feet at night with socks, soak in a foot bath, or soak in a bathtub with it. Elemental magnesium is absorbed at 10x the rate of oral magnesium and is very safe.