Stepper Motors Problem

Hi, I just got a Prusa model tested recently. Its running on RAMPS 1.4, Marlin and Pronterface.

My problem is such that the X,Y and Z motors are working fine when I used the test code. However, when I plug in the Marlin firmware and try to move along the axes using Pronterface, the motors seem to have much lower power. They can't cover the whole axis direction, and their limits change every time. I tried to tighten the belts, but the same problem occurs. Change it back to test code and its showing the motors are okay. Reinsert the Marlin code, test it at Pronterface and its like "lagging".

Do I need to tweak the motor's power or is it a firmware problem? Or it is, in fact another problem?

And also I can't run on Sprinter firmware. This was shown in the Pronterface interface when I upload the Sprinter code:

Connecting...
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In order to debug your printer:

1) Make sure that all axis (X,Y,Z,E) move freely with the power off.
2) Check each of them over the entire range
3) Make sure that the set screws on all gears are tight
4) Make sure that the belts are all tight
5) Get the current spec data on your motors from the people who sold them to you
6) Grab a DVM and set the drive current to ½ the spec on the X and Y
7) Set the current to the spec on Z (two motors, each get ½)
8) Configure configuration.h in Marlin so it matches your printer
9) Compile and upload your version of Marlin with the Arduino IDE
10) Connect with Pronterface and make sure the proper stuff comes back at connect
11) Try moving each axis one at a time, check for:
a. Proper direction
b. Proper distance
c. Return to start
12) Try to zero each axis one at a time, check for:
a. Proper direction
b. Proper stop point

Once you have that part done, you can begin to evaluate how much current your motors likely need.