Transmission hesitates when starting from a full stop.

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Sometimes when I am slowing to and still creeping to a full stop I will hear a clunking noise and then the car will come to a sudden stop. When it's time to move on and I increase engine rpm to go, nothing happens. I must increase over 1k rpm and then it reengages, suddenly, and then suddenly leaps ahead. Other times it just shudders a bit when I hit the gas and then drives normally. Not good!