40x3.6V= 144V you have an intermittently open (or in the process of burning open internally) LED. the flashing is possible caused by the cap charging to the point where the bad LED begins to arc over, discharging the cap to 144V, the arc extinguishes, and the cap begins charging again. the open circuit in the LED is likely a bond wire that has separated from the LED chip, or broken inside the clear epoxy. it's very small and isn't easily visible. with the voltage applied, divide and conquer. measure the voltage across the first 20 LEDs, then the second 20 LEDs. whichever side has the higher voltage across it is the side you're looking for. then divide that set half and measure across 10 LEDs at a time. one half of that will have a higher voltage, so divide it into two strings of 5. once you narrow it down to 5, measure across each individual LED until you find the bad one (the one with the highest voltage wins).