Hi there You did not state the value of the reservoir capacitor after the bridge / zener part of the PSU. If the o/p of this is applied to the 7805 without some large value capacitor to act as a reservoir then the 7805 will drop out when the full wave rectified waveform drops below the min input value to power the 7805 which needs about 1.5volts above the o/p (5V) to sustain the o/p at 5v. Try a 100uF 35 vwk alumin capacitor. Tantalums can sometimes be a little iffy. They have been known to fail catastrophically under certain conditions, like smoking failure.