Turned out it was the "noise" (in this case, deterministic unwanted patterns on some key voltages) in the noisy testmode in which there was a lot of switching going on. Thing of my chip as a spider on a thin web - the thin web is the supply and ground connections. Now, the spider has a fan that's out of control - and that keeps bouncing it all around by blowing air around uncontrollably. The solution, in this case, increase the moment of inertia of the spider by increasing it's mass - here, add bypass caps between supply and ground. Electrically, what you're doing to help out the spider is putting a sink that'll make the air the fan is blowing recirculate back to itself - so there can't be any uncontrollable bouncing. Clear analogy ja?:)
Interestingly enough - on the node where the noisy switching signal was being muxed out - if I connected a long cable, I could make things more noisy - and get the glitches back. The extra current pulled from the supply in transient to work the capacitance of this cable was enough.
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