Sudden Y-Motor Motion Stutter

I have a pretty weird problem happening here. I have a DIY Prusa Mendel that has been running fantastic. Until this morning. I did a print late last night that came out great like usual. I let the printer cool down and went to bed. I did nothing out of the ordinary. I woke up this morning to do another print, turned the machine on, and started the print. All of the sudden the Y-motor is funky. When the motor homes, the movement is smooth but when the print begins, the y-motor movement is VERY choppy. Its at a constant frequency and does it regardless of speed. I thought maybe the driver board went bad so I changed it. Same problem. Smooth homing then choppy print motion. What the heck is going on?!? Please help!

All of the other motors work fine. Also, when the Y-carriage is moving, you can see the belt vibrating up and down wildly. It normally moves nice and smooth. This is boggling my mind. It was a little cold last night and the window was open. Could there be some condensation built up inside the electronics? I just put a hair dryer to the ramps and arduino board to warm them up a bit. Still the same problem...

Did you plug the Y motor into a different controller? Perhaps the idler is jammed? Or the gear on the motor is loose?

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MakerFarm 8" Prusa i3v
RAMPS 1.4
0.4mm E3D v6 for 1.75mm

Thanks for taking the time to help. I checked the idler, it spins freely. I checked the gear on the motor, it was tight. Ran a print and still smooth homing, still choppy movement during prints. I changed the motor to another one I had on hand which I know works fine. the problem IS STILL present. Smooth homing, and then rough print motions. Belt still vibrating wildly. I am beginning to think there might be something wrong on the ramps board. I don't know what to do! This is just another punch in the face from this damn machine. Works fine for a month, then something weird happens!

swap theconnectors of the x and y-motors on the board and then print, if it is still in the y axis it is not your ramps.