Pops, clicks and dropouts during recording or playback usually is caused by the sound card/interface latency setting being too low. Before you check the latency of your Tascam, make sure that you have the Tascam ASIO driver selected in Cubase (Devices, Device Setup, VST Multitrack (LE) or VST Audio System (LE4)). Once you are sure that you have the right driver selected for the Tascam unit, open the Tascam software an raise the audio buffer setting.

When you chance the audio buffer setting of your Tascam unit, do you hear any difference in the frequency of the pops and clicks?