[NASSTY]

Hi Mando, I assume that because you're using AFR heads and 8 T/bs that a Ford EECV computer wouldn't cope and you're using an aftermarket ECU? I'd also assume if you are using an aftermarket computer that's it's probably tuned with throttle position (TPS) as the load input as you've probably got bugga all manifold vacuum with the 8 t/b setup? If yes to these questions then you've gotta get the tuner to enable closed loop. Why? Because TPS cannot adjust for changes in atmospheric pressure and heat soak of the t/b's especially at idle (there's 8 of 'em so even a little growth will play with the a/f ratios) without closed loop enabled. If tuned under MAP (manifold absolute pressure) then it will adjust for atmospheric pressure changes but not for heat soak of the t/bs. Get your tuner to check the end angle of the injectors too as if the angle is too early then unburnt fuel will go straight out through the exhaust valve on the cam overlap. If it still fails after that then I'd look at individual cylinder tuning (assuming your ECU can do this) because it's a fair bet at idle and light throttle your t/bs won't be balanced which would cause a lot of variance of the a/f ratio between cylinders.
Problem is this all takes a lot of time and $$ so some tuners won't do it or they just don't know how. All you need to do it yourself is a wideband a/f meter and some patience.

Good luck with your next test.