Bmw key

Hey guys, I bought a Bmw 320 private sale, it has
been working perfectly fine except recently I was
driving and a sign came up on dash of a image of
key with slash through it, I had another message
displayed on the screen something on lines of if you
stop car may not start again. When I finally did
come to a stop and turned off the ignition I put the
key back in and the car wouldn't start!! The same
image of key with slash appeared on the dash!! I
went back to car after hour or so and it worked
perfectly fine. But again when driving at random
times the same sign would appear. It's happend a
few occasions now and if I get in the car sometimes
I need to insert my fob 10 times +  before the car
comes on! But once it's on it drives fine! No
problems! So basically it's very temperamental! Any
ideas what this could be.