Extruder motor spontaneously stops working

Ok, reprappers, maybe you can help me out here, because I'm out of ideas.
I was printing some parts for a costume, on my prusa i3. It's a ramps/mega build, with one extruder and heatbed, and some modified framework. It's been working beautifully for months, aside from some small bed leveling issues, but extrusion stopped about 20 layers into this print. I assumed the problem was a clog, but then I noticed that the extruder would not retract. After some experimentation, I found that the extruder does not turn at all, even with no load. At most I get a tiny jerk at the first command to move, and then nothing.
Being somewhat experienced at odd printer problems, I tried/concluded the following:

• Swapped the E and Y motors. The E motor works on other drivers, so it's not the motor.
• Swapped the E and X drivers. The E motor did not move(on the x driver) and the X motor did move (on the e driver). So it's not the driver.
• No 'cold extrusion prevented' errors, and the extruder fails to move even when the heater is at temp.
• The motor is getting current, and holds fast when I tell the motor to move. That is, I can turn the axle freely until I tell the motor to move, and then it is still until I cut the power.

so... I'm stumped. My next course of action would be to buy a new RAMPS, which I was planning to do anyway as a spare, but is there anything you folks think I should try before I buy new?

Update: I saw another thread about adding a second extruder to a prusa, and it got me thinking. I moved the extruder heater, thermistor, motor, and driver over to the extruder 1 slot, where they had been in extruder 0. Turns out, extruder 1 works just fine, with motor moving and clean extrusion on a short test print. My guess is that my RAMPS board had a bad connection that finally gave up after all these months of printing. Thoughts?
Also, while I want to continue using only one extruder, in order to use extruder 1, I have to connect the heater wires to D9, where I previously had the fan. How would I go about modifying marlin so I can use the extruder 1 motor control but the extruder 0 power?

In Marlin look for Pins.h ( almost at the bottom of the list ).
There is a section about extruder pins, which you can change to your needs.

When the stepper holds the Position, but doesn´t move, there are not many options of pin-failures. I´d check "step, enable" for cold solderjoints on the Driver,RAMPS and mega board.