Try to use voice modulation. If you feel that a block is coming, think about using voice modulation, e.g. starting with a low tone and moving to a higher one or vice versa. This modulation can be subtle, barely noticeable to the listener. This will allow you to focus on the modulation and not the block. You can also use a slight forced smile while speaking. These are practices from a good book by Lee Lovett. It helps me with blocks.