Usually dropouts are hard to exactly target.
They can usually be associated with the DAT in this case after listening to your sample. Often times dropouts from the DATs can be caused from a few variables:
the DAT heads can be dirty, there could be a misalignment, the tape could have an error in it.
Dropouts do happen when the recorder cannot process the information correctly. Same for diginoise.
It is possible, however, to have a dropout occur on one deck, but on another deck the tape can be "clean"
Phil