Random missed steps in Y axis

Hi,

I have an interesting printing problem. The Y axis is sometimes displaced by a few milimeters. It almost always happens with bigger objects. Smaller ones don't seem to be affected. I haven't been able to catch the exact moment when it happens so I can't describe any symptoms. I have attached a picture of a large object where the skipping happened two times during the printing process (arrows).

1) First I have tought that was a mechanical problem where the nozzle hit the object and displaced it but it should have been a more irregular displacement, so my assumption can't be true.
2) I don't think it is a permanent mechanical error, otherwise it should have been more consistent and happened in every print.
3) Maybe the stepper driver has resetted itself due to heat? I have a heatsink (but the adhesive weared out, so I don't have a very good contact) and also I am using a fan to cool down RAMPS. The heatsink doesn't get hot to touch so I wasn't expecting a reset due to heat (then again maybe it can't perform its job due to a lack of proper adhesion).
4) The stepper driver needs higher voltage (I need to adjust the potentiometer). Normally the extruder and the X motors get hot during print but the Y motor is cold. Can it be a sign that it is under-powered? (Note: I have tried to increase the voltage but somehow the stepper became nonresponsive and isn't working right now. I am waiting for a replacement)

Thanks...

Commissioning, Calibrating [reprap.org]
The incomplete reprap beginners guide. [reprap.org]
Machine Calibration, Triffid Hunter's Calibration Guide [reprap.org]
Calibration [reprap.org]

Calibration of stepper motors [www.youtube.com]
How to measure Stepper Motor Voltage [forums.reprap.org]
Tuning and Speeds – Adjusting the Machine's Axes for Maximum Speed, (Kit Supplier) [solsylva.com]

How to configure microstepping for Pololu A4988, and DRV8825 [forums.reprap.org]
Trim the stepper drivers, A4988 stepper drivers at 0.4V. [forums.reprap.org]
For measuring the stepper driver current you can measure the voltage across the shunt-resistor and use Ohms law to calculate the current values from [forums.reprap.org]
Acceleration vs. Speed [forums.reprap.org]
Calibrate Stepper Drivers – RAMPS 1.4 [bootsindustries.com]

Quote
A2
Commissioning, Calibrating [reprap.org]
The incomplete reprap beginners guide. [reprap.org]
Machine Calibration, Triffid Hunter's Calibration Guide [reprap.org]
Calibration [reprap.org]

Calibration of stepper motors [www.youtube.com]
How to measure Stepper Motor Voltage [forums.reprap.org]
Tuning and Speeds – Adjusting the Machine's Axes for Maximum Speed, (Kit Supplier) [solsylva.com]

How to configure microstepping for Pololu A4988, and DRV8825 [forums.reprap.org]
Trim the stepper drivers, A4988 stepper drivers at 0.4V. [forums.reprap.org]
For measuring the stepper driver current you can measure the voltage across the shunt-resistor and use Ohms law to calculate the current values from [forums.reprap.org]
Acceleration vs. Speed [forums.reprap.org]
Calibrate Stepper Drivers – RAMPS 1.4 [bootsindustries.com]

Thanks for the answers. So , I assume you think it is a stepper driver voltage adjustment problem and not a physical problem....

Your print looks like hell, I just assumed you didn't calibrate "anything"

You are absolutely right but the main reason isn't the lack of calibration. Two of my support structures decided to fall down mid-print and I was away so I couldn't intervene. You can disregard general print quality