It can't be the O2 sensor if it only happens in open loop. It's not reading off it at all then... if it only happened in closed loop, then it'd be a very good possibility, though.
What does the studder feel like? Does it feel like a lack of boost, a lack of fuel, a small ignition cut, some unidentifiable hiccup? Do you notice anything going on with your EGT's, A/F's, or boost when it occours?
I would doubt it being a compression problem if the car runs just fine at idle, where you'd notice it the most. At WOT, the car has very little chance to lose boost out the rings (due to, say, a common crown breaking off), and at idle the vacuum and rotation of the motor is so low that it'd be much more likely to run roughly (BTDT).
How are you controlling timing? It's possible that it's at the threshhold where the ECU may hear detonation and pull timing, causing a dip there. Wish there was a simple knock-link circut that could be made to test this...
I'm going to guess that it's detonation due to low/lack of fueling and advanced timing.