I doubt it will even lift off the ground with a 2000mah battery...hahaha.
..A syma x8c 1200 mah is 160 plus grams by itself...Way too much battery for an x5c...Its possible the FC is going into thermal shut down from too much amperage trying to lift that battery...Im surprised it hasnt burned up already.
A 750mah battery is the largest battery you should use.