so ive been fighting this for like two weeks now and im starting to lose my mind. running a 45x45 stack, speedybee f405 with their 50a esc, 2306 motors on a 5inch stretched x frame. on normal cruising and even mild acro its fine but the second i throw a hard punch or do a split s from altitude the whole thing just drops out and reboots. sometimes it catches, sometimes it just pancakes.

ive checked continuity on all my solder joints which all look fine to me, caps on the battery leads are two 1000uf 35v Low ESR caps soldered pretty close to the xt60. running 4s with a decent 75c lipo so i dont think its the battery sagging that bad. betaflight logs show the voltage dipping to like 13.8 on a fresh pack which shouldnt be enough to brownout anything.

only thing i havent tried is reflowing the esc power pads, one of them did look a little cold when i look at it now with a loupe. could that actually cause intermittent brownouts or am i chasing the wrong thing?

cold joint on the power leads will absolutely do this, ive seen it a dozen times. looks fine to the naked eye, maybe even okay under a loupe, but under load the resistance spikes and thats all she wrote. reflow those pads with fresh flux and make sure you're getting good heat into the pad itself not just the wire. use at least a 60w iron on something like that, the big copper pads just suck heat away and people underestimate it.

also two caps might not be enough depending on how long your battery lead is. if youre running more than like 5-6cm of wire from xt60 to the esc input i'd throw a third cap on there or swap to something beefier. the speedybee 50a is decent but their caps from factory are kinda marginal imo.

had almost the exact same thing on my last build, turned out one of my motor screws was too long and was intermittently shorting against the stator on hard vibration. complete red herring compared to what you described but worth checking if the reflow doesnt fix it. also what fc firmware version are you on, theres a known issue on some older betaflight builds where the vbat cutoff threshold was getting triggered wrong