Analog mean-delay (AMD) method is a new powerful alternative method in determining the lifetime of a fluorescence molecule for high-speed confocal fluorescence lifetime imaging (FLIM). Even though the photon economy and the lifetime precision of the AMD method are proven to be as good as the state-of-the-art time-correlated single photon counting (TC-SPC) method, there have been some speculations and concerns about the accuracy of this method. In the AMD method, the temporal waveform of an emitted fluorescence signal is directly recorded with a slow digitizer whose bandwidth is much lower than the temporal resolution of lifetime to be measured. We have found that the drifts and the fluctuations of the absolute zero position in a measured temporal waveform are the major problems in the AMD method. We have also proposed dual channel waveform measurement scheme that may suppress these errors. It is shown that there may exist more than 2 ns drift in a measured temporal waveform during the period of the first 12 minutes after electronics components are turned on. The standard deviation of a measured lifetime after this warm-up period can be as large as 51 ps without a proposed scheme. We have shown that this error can be reduced to 9 ps with our dual-channel waveform measurement method.