Possible causes:

1) loose belt
2) insufficient voltage on the stepper
3) stepper needs to be replaced
4) too high movement speed

Solutions:
1) tighten belt
2) increase voltage on stepper slightly via firmware parameters
3) replace stepper
4) print slower (and also move slower)

If 4) then adjust movement speed to be equal to print speed first, this will give very little time penalty and may solve your problem. If this doesn't help, try lowering both print speed and movement speed.

1) is easily checked. Then I would try solution 4) first. If it helps, then 2) may allow you to increase speed again. If all fails, then 3) is your final recourse.