What could cause mouth opening and breathing noise like darth vader (not snoring), with teeth clamping shut (not grinding) a few times while sleeping?

Test: You may have airway insufficiency resulting in sleep apnea. See a Pulmonologist. A sleep study may be in order. If you are grinding or clenching, or if you have a narrow upper jaw and/or retrusive lower jaw, an Orthodontic examination should be performed.

Obstructive apnea: These symptoms are extremely common in people who have obstructive sleep apnea, See your doctor for PSG or home sleep study to rule out sleep apnea.