The authors have measured the positron lifetime as a function of temperature from 80 to 300 K for the Bi(Pb)-Sr-Ca-Cu-O superconductor (the Bi(Pb)2:2:2:3 phase). They have observed that the positron lifetime results display three narrow valleys near 120, 140 and 160 K and two wide valleys near 240 and 270 K. The anomaly in the positron lifetime indicates the structural instability of the Bi2:2:2:3 superconductor in the normal state, which may be caused by structural changes, such as the softening of the lattice, or the motion of vacancies in Bi-O layers. The authors have also carried out positron lifetime measurements as a function of temperature from 80 to 180 K in the Bi2:2:1:2 superconductor; on comparison with the results for the Bi(Pb)2:2:2:3 system, they found that adding Pb impurities can stabilize the Bi2:2:2:3 phase.