Okay, with individual coil packs, a tad harder to determine.
Still can be a coil pack breaking down.
I have this old Snap-On hand-held spark tester, all you need to do is get the end close to the spark source and see what it's doing.
You adjust the wheel to where the light stops (kind of like a "squelch" knob).
If I was doing it, I would have someone load it up a bit in gear and check, comparing to idle readings.
Usually, if an O2 is bad, it will read out-of-spec and trigger a code (non-Jag).
You said the plugs look good.
Does that mean porcelain color at tip?
If they're tan, probably not lean.
If they are pearly white, with maybe splash spots, could be lean.
Your original post sure looks like ignition breakdown.
Under load, you can push through (actually, I would bet you are getting to a "less load" condition).
A vacuum gauge would tell you. Many the time I stuck my 4" gauge under the wiper blade, and took a car out for a drive.
You can see "load" and where it breaks down.