A few years later...but I figure someone might find this thread while searching for help as I did.
Jts1957's advice was right on for my dvr670 unit. Mine had a similar problem. It would intermittently shut off during tape play and then eject the tape when I powered it back on.
Using the clue of reel sensor, did some reading, disassembled my unit, and it is now working properly. Info I found on the web suggests that the reel sensor would be under the take-up reel, but it is not on the dvr670.
On the dvr670 the gear teeth of the take-up reel drives a thicker (taller), smaller diameter gear which is towards the middle of the drive. That gear drives a third gear which has two cutouts in its body. This third gear is overhung by limb from a clear plastic optical prism. The prism is held in place by one screw and is easy to find. It's near the rear center of the cassette space and has a vertical post in addition to the limb overhanging the sensor gear.
After removing the screw and prism, the LED will be found under the prism. With the prism removed the sensor gear can be lifted off its spindle. Under the gear is a hole with the sensor down there. The hole is too narrow for a q-tip.
So the LED can be reached with a q-tip for cleaning. I had to use a small paint brush to clean the sensor. But I think the real problem was some accumulated schmutz on the under side of the limb of the prism. It wasnt obvious, but close examination showed that the clarity of the limb was compromised by a thin layer of something.
In all cases, q-tip or paint brush, I cleaned with 91% isopropyl alcohol.
The problem has not recurred yet.
So thanks to jts1957 and I hope these extra details help someone.