It sure sounds like voltage drop in the wiring. If you have 140 Watts on a 12 volt system, that’s 12 amps. That is a lot of amps! If your run is 100 feet, that’s 200 feet of voltage drop (assuming all the load is at the end, which it probably isn’t). If you used 12 gauge wire for 200 feet, that is about 0.3 Ohms, which doesn’t seem like much, but multiply that by 12 amps and you get a voltage drop of 3.6 Volts. LEDs have a nonlinear output characteristic and require more voltage when they are hot. My guess is that you dropped the voltage to the point that they go on, but immediately heat up and turn off, which repeats in a thermal relaxation oscillation.