This reminds me of a problem I had with outdoor garden lights back when we lived in Las Vegas. I had a long string of LED lights, and
After I connected the 12th fixture, the six or seven fixtures farthest from the transformer all started flashing on and off together.
@JWT_Kottekoe explained that my problem was probably a voltage drop.
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.
To the OP: are the lights in question new? Could they all be on the same stretch of cable? If so, it could be a voltage drop. But if they’re existing ordinary fixtures that only just started acting up like this, it’s probably what everyone else is saying.