It seems to me that a poison pill would make the changes in difficulty larger not smaller.  There are 2 obvious ways to smooth out difficulty shorten adjustment period or make only fractional adjustments.  I think 30 or 60 block adjustments will work fine.  Anything > 5 min and < 1 hour seems pretty solid.  Fractional adjustments would also work if you did say a geometric mean of the previous 2 targets with a double weight on the new target.  This can mess with the time a bit.  You probably could even use the same concept as pools use for PPLNS on difficulty adjustments.