Digado has a good suggestion that I have added to my notes on using the 670.
Unfortunately, I have seen that the DVD recording will have the quality of what it receives. I had a tape that was recorded in 6 hour and wanted to get a DVD recording of its first hour. I saw the tracking problem on the tape as it played and that's how the DVD recorded. In this case, since the program was still on TV from time to time, I rerecorded it on tape at a 1 hour speed and the dubbing came out without tracking problems. I believe that tapes done in 4 hour speed or less will have the best picture quality transferred. I, too, have always recorded at the EP speed.
I also see the Toshiba VCR section...will play a tape without tracking problems if: 1. the tape was originally recorded on the Toshiba or 2. I record it on a different VCR but rewind it on the Toshiba.
Other than what Digado suggested (which could be the best way providing you have another VCR to use), the only thing I can suggest since you mention it happened in the middle of the tape, is, for you to play the tape on the Toshiba first and see if you have to do any tracking adjustment..then rewind it on the Toshiba..and see if it tracks okay. This would take sometime but I don't see anything shorter. I have a Sony VCR and with certain tapes the Toshiba has a tracking problem, other tapes it is fine.
The 670 manual says on dubbing the picture may be distorted due to auto tracking and suggests playing back the tape until the picture is stabilized and then set a starting point. Sounds like there is no way to turn off its auto tracking. So the other work around would be a second VCR to the DVDR section of the 670....or to have to watch the tape to see what the recording will look like. If your tapes have sat for awhile....another thing to do is FF and REW the tape so it settles okay after having sat with no movement. This will help tracking problems too.
Another suggestion that I have done is do the recording to a RW type DVD so if you are unhappy with the result, at least you can reuse the DVD to try again. I did that with the 1 hour tape I had tracking problems with when it was recorded in EP. After I rerecorded it as SP..1 hour.. the dubbing came out perfect.
Sorry, I don't see any easy answers except that idea of a second VCR maybe tracking better. Even then you should see the picture on that 2nd VCR cause if it has a tracking problem also, that will be transferred to the DVD you are recording to. But I do know the Toshiba can have playback tracking noise that has to be adjusted and all there you can do is hope the noise is at the start only so you can adjust it quickly without monitoring the entire 6 hours.
Good Luck....hope something here helps.