Adafruit IR Break Beam Sensor - 5mm LEDs [ADA2168]

Installed the pair 35cm apart with separate 5V supply and 10k resistor for the pull-up on the control line. Signal is read with an Arduino Uno.But the behaviour is intermittent. Bought another pair, same thing.With nothing blocking the beam it produced a HIGH output as it should, then LOW when blocked. But over a few minutes with nothing blocking it, it would go LOW and stay there for a while then start working normally again. Seemingly it works, or doesn't, randomly.I stripped out all the wiring so it goes direct to the bread board now, tried it with a physical 10k pull-up and then with a software configured pull-up on the Arduino input. Same story.At one point, waving my hand **behind** the transmitter in a certain place would reliably send the signal LOW (blocked).For some reason I had assumed this was using pulsed IR e.g. at 38 kHz. If it is then it's just a bit useless. If it's not then maybe I'm just getting ambient IR interference. Either way I can't use it.