sounds like your tranny is dying. check your tranny fluid level, preferably you want to flush and refill. if that dont solve it, your torque converter may be toast.

IMO, from what i can gather is that the fluid is not able to flow properly to spin inside the stall converter and eventually get the tranny and engine spinning at the same speed to lock up and drive the wheels.

if you need a clear explanation of how an autotranny works so that you can imagine my diagnosis, check out www.howstuffworks.com