I have begun using an old Prusa i3 donated to our school.
I achieved one full print with it.
The next two prints went for 20 minutes and then something strange happened:
-- the temperature setting reset to 0 and the actual temperature began to plunge
-- the print head stopped moving but the LCD readout on the machine kept updating with new X and Y coordinates, as if the machine was still printing.

I tried pausing and restarting the print, to no avail.

I'm using Slic3r.
I checked the GCode and there are no M104 codes except at the very beginning and the very end.

Very bizarre behavior!
Any ideas???