Does this happen when you put it in reverse or drive, or when it is shifting between gears? In any case, i would check your transmission fluid level, it has been my experience that our transmission is very touchy oncerning fluid level. It might be possible that your stall speed is off, i dont really know.
From the bentley manual:
"hard jolt when shifting from N to R or D"
"a. idle speed too high"
"b. ATF level low"
"c. Faulty transmission range switch"
"d. Internal transmission fault"