what could cause missing and roughness when idling. only does occasionally

More than likely this is one of a couple things, so we need to start with crossing things off the list.

Let's start with checking for a vacuum leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If there is no vacuum leak, then the most likely issue is a purge valve issue.

Basically, what happens is when the purge is supposed to be closed the vehicle will run poorly because it is too rich due to the purge being stuck open or vice versa if it is stuck closed it will run lean.

So, we can monitor the o2 sensor data to see if this is going rich or lean and that will help us figure this out.

I am attaching the testing below that will help with this. Please run through this and let me know what you find.

Thanks