Belt tension — the Z-axis should be guitar string tight (but careful not to bend the motor shaft): http://docs.carbide3d.com/assembly/shapeoko/xxl/step-5-belting/ on deep cuts it may help to remove one spring from the Z-axis temporarily. Some folks have found it helps to remove the M4 Z-axis tension bolt, apply a thin bead of threadlock along the length of the threads, then reinstalling it after it has dried.
It’s also possible you have a belt with a flaw or some sort of damage.
For electronics, it’s possible you could have an intermittent wiring fault (but those are usually accompanied by awful noises), or have some sort of electronic issue with a stepper driver.
The other possibility is feeds and speeds — what sort of materials are you having trouble with using what endmills?
Did you make a hello world (Shapeoko) print after you assembled your machine? If you did make one, can you post a photo of it. We (experienced users) can often tell a lot about the machine from that photo.
ALL your belts are probably too loose, and check ALL the set screws on all 4 pulleys. The set screws (1) must be on the motor flats and (2) need to be very tight. Here are a couple of photos to help: