[G6ONEDave]

According to the Technical manual
"Auto Stop Mode" is as follows; The mechanism enters Auto stop mode in the following conditions.
a) IN REW and REV Search modes, the tape travel reaches the beginning of the tape. When the leader tape is detected, the tape is forwarded in the normal direction until it's leader portion is not detected, and then the mechanism enters the Stop mode.
b) In the case of the Pause/Still mode continues for more than 5 min and 25 sec.
c) In the Play, REC, Search and FF/REW modes, the take-up spool stops rotation for 3 secs and more.
d) In the Play, REC, Pause, Still and Search modes,the drum motor stops rotation for2.5 secs and more.

"3 Auto-rewinding mode" is as follows; a) The mechanism enters Auto-rewinding mode when the tape travel reaches it's end in the Play, REC, Search, FWD and FF modes.

The machine sounds like it is reacting to a problem with the tape transport system, hence my previous comment about the back tension band.

You could try slightly pushing the tension arm back by 0.5mm to see if the machine will get to the end of the 3hour tape. I suggest that you use a sacrificial tape for this in case any damage is done to the tape. I would also check that the supply spool rotates freely when there is no tape in the machine.

Dave