I exclusively experience silent and audible blocks. I've have never shown involuntary prolongation or repetition. The hardest sounds for me are words that start with vowels, or words that start with letters such as 'r'... Some techniques I've used to overcome a block is to purposely and softly prolongate the first syllable. But most of all, remember to breathe. When you feel a block happening, rather than suffocating yourself trying to break through it... Stop.. Take a deep breath, and try again. The deep breath can also help you to relax. Try to concentrate on relaxing your vocal chords, mouth, and shoulders when you take the breath. This works a lot of the time for me, but sometimes I have to repeat this a couple of times depending on the stress of the situation.