You will have to visually see what is going on with the slide mechanism. It has some sort of spring ratcheting type gear system, so mine will jump when it reaches the end of the travel, I've learned to release the button as soon as it reaches the in/out.
To access the drive mechanism you will have to remove the platform cover under the bed, there are ~4-6 screws holding it on. Remove the screws and lift it up, you should be able to see the slide mechanism.
Possible cause of your problem is loose screws letting the mechanism jump. Other thing to look for is the slide gear is stripped and/or a shear pin broken.
If you can any identifying numbers post them and we might be able to zero in on the make/model. Get the number off the motor, it will help.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi