Extruder motor stalling

I have been printing for about a week now and made 5 moderately sized prints that came out fine. (All smaller than 50x50x50 mm)
I set a print going last night, which was a gear 80mm in diameter and 7mm high, with 0.3mm layer height. The ETA was about 1.5 hours.
It was printing fine until about 45 minutes in (layer 15), the motors got hot, and the extruder motor started to stall (motor humming but no movement).
The extruder would retract fine, and then move forwards up until the amount it had retracted, and then stall and hum with no movement. So as a net result, no filament gets extruded.
So I turned off the printer and left the motors to cool overnight, thinking that it would be fine once the motors had cooled, but now whenever I print, the extruder motor starts to stall after only 5 minutes or so.
I can extrude material by turning the gear by hand, but it feels a little stiffer than it should.
I know you guys are going to tell me to turn up the current to the motor, but:
1. I am scared to do this since once I fried a motor driver just by turning the pot a little.
2. Why has it only started to do this now? I made 5 medium sized prints before with no problems.

I'm printing with PLA at 175c, bed 45c

Thanks guys

Edited 1 time(s). Last edit at 05/01/2013 11:28AM by KarlB.

I had a similar problem twice. once, the motor got hot and sofend the gear so the grub screw pushed out. Second time the grub screw was just loose.

Could either of those be possible?

Tightened the grub screw. It took about 1 1/2 turns to get to the point where it could no longer turn, so maybe it was in fact a bit loose.
Re-printing now and currently on layer 5, so things look good.
I will eventually aim to get a fan on the stepper motor to keep it cool, but until then I'll pause the print every half hour or so and leave it for 5 mins to let the motors cool down.
Thanks for the help.