Car shutting off

My car has been randomly & infrequently shutting
off. I will be driving & if I brake maybe too hard the
battery light will come on and the car won't move.
The power in the car will still be working (like the
radio) but it's like the engine shuts off. When I put
the car in park, turn it off and turn it back on it's
usually good to go. Yesterday though when I put it
in park & tried to turn it back on it just made a
clicking sound and would not start. I had the car
jump started & it came back on. But then it
happened again with the clicking noise and when I
had it jump started it would turn back on so I had to
have the vehicle towed. Today my uncle who is a
mechanic came to have a look & when he put the
keys in the car started right up and he took it for a
drive around the block & it was "fine." He took a
look under the hood and was saying he doesn't see
a problem because both my battery and starter look
good. I'm not sure what the problem could be.