The new LED strobes while the old incandescent light works fined. I replace one of the two bulbs on the motion detection light fixture with a 150w incandescent flood light bulb....and the LED flood light stopped flashing/strobe. Why will the out door fixture work fine with one LED bulb and one incandescent bulb....but only flash with two LED bulbs?
Most of those old motion detector stalk light deals are modular. You can open them up and change just the motion sensor, for instance.
That's exactly what you can do. Change the motion sensor to one that does require a neutral wire. That means the sensor will be powering itself with its own local power supply, and won't be leaking current through the bulbs.