Y axis moto going crazy ?

Not quite sure what you mean by "acts crazy"?

But I am guessing by your comment about the Y losing its reference that you might have a problem with the Y endstop. All it takes is for the Y endstop wiring to go open circuit (even intermittently) and the Y axis completely loses its reference. When this happens only option is hit the emergency stop quickly.

You can check the status of the endstops using the gcode M119

http://reprap.org/wiki/G-code#M119:_Get_Endstop_Status

If the Y_min endstop status shows as "TRIGGERED" when it actually is not, then you have a break in the Y endstop wiring somewhere.

 

Recheck all of the connections to the RAMPS board and make sure nothing is loose. Something loose could be getting shaken during a print and triggering a false positive on an endstop.

 

If you are only seeing a jump of less than 1 cm then it is possible you just have a loose pulley on the Y Axis (or maybe a loose belt). Check the set screws on the Y Axis motor pulley are tight and the belt is tight. It's good practice to check these anyway.

 

Ziggy is spot on. Mine came with a *lot* of loose belts and pulleys (all of them actually) but yours could have worked loose.
I used loctite on the pulley set screws... the belts you have to tighten periodically if they stretch any (once you get them tight).

 

If that set screw is tight then you have one of two thing happenings and the first is more likely
1 the belt is slipping
2 the motor is skipping steps

the reasons I think it is a belt slipping is that the direction of slippage seems to be in one direction just gradiating a bit each time a skipping steps issues would show more as random in direction.

if you pluck the belt under the table ( the Y axis belt ) what kind of sound do you get, the reason I ask is that a belt that is too tight will do almost the same thing as one that is too loose.

When plucked like a guitar if the sound you here is actually a sound that has pitch to it the pitch should be very low if it is in any way high it is too tight.
If the sound is just like a thud noise then it is too loose it needs to be tighten a bit but be careful the difference between a no pitch thud sound and a low pitched musical sound is very small it can easily be tightened too much.

If it is too tight(higher pitched sound) the entire bed becomes too hard to move as quickly as is need to print reliably if it is too loose then the motor turns the right amount but the belt slips

The second thing has to do with the amount of power being sent to the motor it should be very close when the printer arrives but it can be off this is much harder to correct as you will need to use a multimeter on the stepper driver on the ramps board to check things out.
A quick test that can let you know if too much power is being sent to the motor is to check the temp of the motor itself during the print

All the motors should be only slightly warm while operating if any of them are exceedingly HOT then you have too much power going to them and they can burnout so check them all but be careful in case any of them are too hot they can burn you if it is set wrong.