Besides low magnesium, low blood sugar spells is a common cause of twitching muscles:

http://symptomchecker.webmd.com/mult...ns=49|50|49|50

I read a book on hypoglycemia once that claimed up to 70% of people having a low sugar attack will have muscle twitching.
Low blood sugar also causes feelings of anxiety, and since that topic comes up here often, it needs to be addressed again.

The best way to deal with low sugar attacks is to clean up your diet, avoid white carbs and sugar and eat balanced meals with some protein in each one, and lower glycemic vegetable or fruit carbs. Some fruits are lower than others. Bananas are high and peaches much lower, for example. Investigate the Book or online by Dr. Barry Sears who developed The Zone diet. Eating this way is rather easy, and controls blood sugar as well. The Zone encourages the consumption of Omega-3's also, and limiting inflammatory high amounts of Omega-6's. A high inflammatory diet is going to release cytokines from the Cox-2 enzyme and those may trigger the twitching.