hmmm ok.. i was gonna ask you at which point the cutting out happens and you've beat me to the punch

that uphill gradient tells me something, the fact that the problem is more pronounced uphill tells me that it's a "load" issue. by that i mean, the battery is unable to supply the requied power to the motor and reaches its limit and cuts out.

if you had a cycle analyst hooked up, i bet you that you would see your amps and wattage go high just before your battery cuts off. and by "high" i mean whatever cut out point your battery can't afford. that number may differ based on the battery. in your case that limit may be reached because your battery reached its limit and that may be due solely to the capacity of your battery, or the battery reaches its limit sooner because one or more of its cells isn't properly balanced or the bms itself is busted. whatever the root cause the scope of the problem is quite clear at this poitn, it's causing the bms to believe it's attained the limit. in movie terms, "you [the battery pack] can't handle the truth !!! [amperage]"

how much did ya pay for the bike? if ya don't mind my asking (use of ya, means i'm talking money, lol)

all levity aside, you need to consider how much you're willing to invest in this. are you looking to get a good performing bike etc? maybe i should just read your post ok so that tells me something. well, i was of the same mindset when i started and had a budget cut out , so there ya go.