The M96 is known for broken valve springs.. That could be the issue. A compression test and leakdown test would be the first diagnostic that I'd carry out.

Jake,
Since a broken spring will affect the leakdown test, does this means that the M96 heads have a single valve spring on each valve?
.

Yes single spring

A compression test and leak down test may not show anything - the valve will seal shut with compression or supplied compression via a leak down test as long as the valve is in decent shape.

Still, they are quick and easy tests, worth doing. I think ultimately you will have to lift the rocker covers though.
Sam

Just to give a final update - after tending to some maintenance (2 rear wheel bearings, axle boot covers and repacked cv joints, brake and clutch bleed, air filter), I was able to take the car out for the first time today since the original posting.

No more signs of trouble. Took it up to 6500 rpm, sounds and drives beautiful. It appears that indeed the culprit was a loose spark plug. Whew...