The receiver is just looking for any variation in light levels, and it will also operate when there is noise on the power supply. That will make it fail to detect a break in the beam. Power supply noise could come from the power supply or from the transmitter. Either way, you should isolate the receiver from that noise with something like a 10 Ω resistor in series, close to the receiver. That, along with the 1000 µF capacitor in the receiver (C5, shown near the buzzer) should remove noise at the frequencies that the receiver is sensitive to.