Having intermittent power loss to my flight controller during flights. FC shutdown causes the quad to drop out of sky unexpectedly, usually caused by lost power to the FC from cold solder joint, loose cable, or voltage spikes. Cold/dry solder joints to ESC means the quad doesn't respond to TX stick for a brief moment or just drops out of sky. Checked all my power connections and they look okay visually. Should recheck solder joints and wires for continuity - sometimes when using JST connectors with silicon-coated wires the wire may be broken at the crimping point. Any specific areas I should focus on first?

With battery connected you should get all 5 tones - first 3 are ESC's booting up, second set is when FC checks in with ESCs. With no power indication on anything besides ESCs it sounds like wire harness between ESC and FC is missing or pinned incorrectly.

Make sure all solder joints have the liquid silver look and are reliable. Bad solder joints may cause FC, RX, ESC or Motor to lose power, prevent signal from FC reaching ESC or SBUS signal from RX reaching FC.

Try disconnecting everything from your FC (camera, VTX, receiver). It's possible something is shorted and pulling the 5V supply down. With only FC and ESC connected, see if you get correct beeps.