surging at idle

[boxster_nut]

97 boxster with 33K miles. I recently changed the oil and filter. A day or two after this job, I notice the engine surging at idle - dropping down to about 300RPM then jumping up to maybe 1100 RPM, but it idles normally about 50% of the time. Sometimes it surges, sometimes it doesn't.

Turning on the A/C bumps the idle up to about 700 RPM and I haven't noticed any surging with the A/C on.

I checked the oil level and noticed that it is a little above the full mark.

I don't know if this is coincidence with another problem or related to the oil change.

Doing some searching about the surging and the most cited answer is to clean the TB and replace the Air Oil Seperator (AOS). I just don't want to throw parts at it, so how can I tell if the AOS is defective?

I will, however, remove and clean the TB as it has never been done.
Insight? Suggestions? Much appreciated.
J