Why do my prints keep sliding?
As Chuck mentioned, if the belt drive gear isn't installed properly (set screws tightened in the wrong order), it can work loose and it will torque back and forth and eventually spin loose.
But those shifts also look symptomatic of dry bearings - what type of grease did you use before you installed them? If not dry bearings, then something causing the to seize. Bearing holders too tight, for example. On Mk3's the extruder case can crush the bearings if the case screws are set too tight, and the bearings begin seizing on the rods.
For this though I have had amazing prints and then all of a sudden shifted prints, then good prints, and so on. Why all of a sudden would it do this. And can someone supply an image of which screws and which belt yall are referring too?
Go back to your assembly manual, review the X-axis assembly instructions. Reread the bit about attaching the belt and gear that fits on the X-axis motor. The key is the bit about tightening the set screws one at a time, flat first. If you don't follow the instructions they will come loose.
As for why and when bearings will seize up - actual travel speed matters; room temperature matters; duration of print matters. Hard to say why it chose now. Regardless, it is happening and you need to deal with whatever the cause is, right?
One other thought on "why now" ... some prints will have areas where infill isn't supported perfectly. So layers will curl at these points, and the curl is usually upwards (thermal effects). These curls can lift far enough the nozzle will contact them as the extruder moves along. This stalls the motor and causes a shift.
The other cause - when bearings seize and stall the motor - can be due to all of the factors I listed in that prior post. To check for bearing issues, power off the printer and manually push the extruder left right stop to stop. It should move freely and without any sticky spots. Even one place in the movement that is sticky is a problem that needs addressing.