Audi A4 difficulty starting

Sometimes car won't start after it started
successfully several times right before...the after
turning the ignition and letting it crank on
continuous tries, eventually the engine grabs hold
and it starts up. Than it works for the next few days  
without issue. Then one day it works for a couple of
short trips, and then doesn't start again randomly.
The same scenario repeats. Usually when it's not
working, eventually you can hear the engine trying
to grasp, until finally after multiple attempts it starts
running. Any thoughts? Thanks