The motors will only day about 10A each with 11v battery, assuming you at rising 10x45 props so your battery should be able to handle that. However how hot do your motors get after a flight?
The voltage drops what only a guess as it's usually the cause of this, especially if the voltage drops with increasing throttle.
Otherwise it could just be a dodgy power module, the o to way to check is to mount your props upside down and/or strap your quad to the ground and connect a voltmeter to the battery while you power it it through the throttle range. If you do this just be careful not to get anything caught in the props!