I've been told that while i'm sleeping i'd all of a sudden 'spaz out' or have a sudden jolt whilst sleeping. what's going on?

Hypnic jerks: Probably hypnic jerks and periodic limb movements of sleep. These are worsened by some meds, caffeine, stress, alcohol, sleep deprivation.

Arousal: Your "sudden jolt" could be an arousal. This is when your body has stopped breathing and in order to get the breathing started again your body moves enough to slightly wake you and get you to breathe again. Actually as long as there are not at least five of these events per hour it is considered normal. If you have excessive daytime sleepiness also, you should see a sleep physician.