I tried some magnesium oil I purchased from Swanson Vitamins. This oil seemed to be more salt than anything and caused the same problem with me. It had all the relaxing effect of gulping down a couple tablespoons of salt. Heart was racing, chest pains, etc.
This was the opposite of what I expected of a magnesium supplement. I went back to my Ionic Fizz Magnesium supplement for that great relaxed feeling. Its a bit pricey, but still best for convenience and absorbability (if that's a word).