I have stop extruding in middle of print issue before. There are three potential cause of these.
1. The fan not cooling enough, and after long time printing, the heat accumulate in hotend and cause PLA to expand and jam the throat of the hotend. You can tell by unplug the filament when hotend is >180C and see if the end of filament is bigger in diameters and the length of expansion is longer than 5mm. The accumulation of the heat takes time lead to delay on the jaming.
2. Repeated retraction lead to Grinding Filament in extruder, this issue can be resolved by change in retraction setting or simply rotate you model before printing. You will find broken filament at extruder position and the grinding takes time lead to delay on the jaming.
3. Extruder overheat, it takes time to accumulate heat, and when temperature raise, PLA become soft, therefore you have trouble to extrude. You can dignose this my feeling the extruder motor temperature after 30 minutes printing see if it gets warm or hot. You will also notice noise when trying to extrude (not click noise, but emits a high frequency noise). If your Jr. is ramps converted, adjust the motor current on E motor drive can solve the problem. If your Jr. is everything stock, you can only solve this problem by add cooling fan to the extruder.
In any of the case, replace the extruder motor of same model are unlikely fix the problem, you will have to use extruder that can handle higher motor current.
(Da Vinci 1.0, Jr. 1.0 RAMPS, miniMaker) X4, (Creality CR-10S, CR-10 mini, Ender-3) X4, Anycubic MEGA X4, Anycubic Chrion X1, ADMILAB Gantry X2 (MonoPrice Maker Select V2, Plus, Ultimate)X4--Select mini X1, Anycubic photon X4, Wanhao duplicate D7 X1.
iNSTONE Inventor Pro X2, CTC Dual X2, ANET-A8, Hictop 3DP-11, Solidoodle Press, FLSUN I3 2017X1