Welcome to NeuroTalk:

Please make sure you are using a magnesium that is absorbed.
Oxide form is not, and will not do much of anything for you.
You need a good chelate, like gluconate, lactate, malate, or glycinate. Aim for 200mg elemental which is 1/2 RDA, or use SlowMag, twice a day (it is chloride in delayed release).
You can get the SlowMag at WalMart.
Oxide is really common and still out there, and people think it will do and it will not.

You could try soaking in a tub of epsom salts, in lukewarm water,
and that may help... the magnesium will be absorbed thru the skin directly in the areas you are having problems with. Use about 6oz in a bathtub. Don't use really hot water, as it may sting.

Some myoclonus comes from higher in the spine however.
So a soak won't reach there as quickly.

Many people may jerk when falling asleep now and then, but regularly is not the same thing.

Are you taking any Benzodiazepine drug at night or during the day? When they wear off there can be mini-withdrawals, which include jerking like this, or cramping. (Xanax, Valium or
Klonopin)