This is an issue that I too am suffering from.
To put it into my words; typically, I have a series of one hour long DV tapes to edit, of dreary family scenes so I split it every 10 seconds so that chunks can be deleted. However, during editing, the first frame is then interposed at each of the breaks. The thumbnail for each clip then maybe, or not, becomes the first frame of the tape.
It can be removed by careful manipulation of the editing tools, but is very time consuming and irritating.
I'm sure that the issue compounds itself with successive edits, that is, more of the clips become contaminated in time.
I'm using Videopad V 7.0