The step motor starts the movement but does not complete

Hello, i am using repetier host to test the motors, with a4988 drivers in Ramps 1.4 and a mega 2560 board.
I set the voltage in the a4988 driver.
In the z axys both motors seems to work ok, but x and y the motors does not complete the movement and make noize.
I tryed to change the motors for z axys and all them work fine.
The extruder motor seems to be ok too.
If I lower the voltage in the driver, the movemente seems to be correct, but I can stop the movement of the motors easily by holding the pulley.

I suspect that the problem is my power source. I am using an old atx delivering 18A and 216 W in the 12v wires. But the problem don't change even if i wire only one motor in the Ramps
Can Anyone help me?

I have found when a stepper motor makes a grizzly grinding noise it means that a connector on the ramps boards is not fully seated into the socket. Like for instance Y goes forward but growls when it does. It means some of the wires are making contact and some are not on the plug connector. Check to see if this is the problem first.

If the motors work fine on the Z axis then it might be the feedrate. Halve the feedrate for the x and y in the firmware then give it a go