Hi. I am not a doctor, but I do have some potentially useful information regarding this subject.
From what I'm reading in these posts, you all sound like you're experiencing propriospinal myoclonus at sleep onset, and its generator is thought to be within the propriospinal tracts of the spinal cord rather than the brain. There are a few potential causes for this condition, but it can also be completely idiopathic. Potential diagnosable causes include tumors of the spinal cord, a few types of cysts within the spine, spinal cord injury, ruptured or herniated discs putting pressure on the spinal cord, degeneration of the myelin sheath caused by n2o induced b-12 deficiency, infections, etc...
Long story short, get the following tests done:
Blood tests for iron, magnesium, and b-12 deficiencies. Also test thyroid stimulating hormone levels to rule out hyperthyroidism.
EMG to rule out neurodegenerative disease.
If all else fails, see a neurologist that specializes in movement disorders, and have him/her do an MRI of the entire spine. If nothing correctable is found, the symptoms can be managed with clonazepam, valproic acid, levetiracetam, and/or primidone. Clonazepam is typically the most effective thing, but because of tolerance issues, your dose will have to be tapered up to a maximum dosage your doctor is comfortable with, then you will have to switch to another medication for a period of time to get your tolerance back down. Clonazepam is also a schedule 4 controlled substance, so be prepared for your doctor, pharmacist, and insurance companies to be a pain in the ass about it; they tend to act like you're the zodiac killer, and they're handing you a loaded gun.