If you have weaker and stronger layers in the same print, one possible cause is a fluctuating extruder temp.

There are two possibilities here:

• Nothing is mechanically wrong with the printer, but the control loop to maintain your extruders temperature is not well tuned. You can fix this by running a PID tune. Just throw a "u" on the end of the line to write the values to memory
• You could be experiencing an intermittent failure, likely in wiring, for your thermistor or heater

Vertical walls will also typically be wavy in this case because you'll have more extrusion flow at hotter temperatures and less at lower.