Getting a mineral test from a qualified lab can make a big difference. Minerals are tricky in that you up one which depresses another. Testing provides a window to see levels, personally I like TEI (Trace Elements) and no I am not affiliated with them.
Magnesium metabolism is perhaps the trickiest of the minerals since it has many interactions and is very sensitive to endocrine changes. Different people respond differently to the many forms of magnesium. Muscle cramping, decrease stress handling, cardio problems, sleep, mood and energy can all be affected by magnesium. Immune function is weakened by mag loss.
Calcium plays an integral role with magnesium in that they should be balanced, testing gives a perspective on this.
Mag threonate may not be the best and the topical forms (mag sulf) may not be either, UltraMag provides a balanced form of mag which some people respond well to. Good reports comes from Mag lysinate as well.
Palpatations are generally regarded as a disruption in electrolytes which as you know mag is one of those, potassium, sodium and chloride are some of the others. sodium is generally considered to be a bad mineral but plays a key role in nerve signalling, potassium regulation, alkaline balance and other things. Too much potassium can alter sodium and vice versa.