Measuring Output Ripple voltage in Scope
Power supply ripple is conventionally measured with the 20MHz bandwidth filter engaged.

As mentioned in the comments, probing technique is critical.  You can't use a long ground clip and hope to get a true measurement.  You have to keep your ground as short as possible.  One of the best ways to do this is to wind bare wire around the barrel of the probe and tip and solder the connections across the output cap as shown below:



Then the ripple should be periodic, so you should be able to trigger on it if you AC couple the scope or add a DC offset so you can get to an appropriate scale.  (See Peter K.'s comment above.)