What the ????

[Old Kiwi]

My car is hesitating when accelerating. We all know an engine needs more fuel when accelerating. Looking at the attached log, most of it looks ok when I first drove off (1st through to 4th), except when I accelerated from 50 to 100 km/h and changed into 5th. Ignoring the odd but generally close enough O2 sensor signals, the STFT for the left bank leaned out because the O2 told it to. That would be ok except I'm trying to accelerate! What's even more strange is that the LTFT is correct for accelerating, although it came from quite a lean state to do so. It suggests a coil or plug is breaking down, but unfortunately we can't see individual cylinders with these ECUs. Are my conclusions correct? Appreciate your thoughts.

[Old Kiwi]

I pulled the plugs from the left bank last night. 5 is a little bit dark, 6 & 7 are good, 8 is ok but has some funny yellow colouring (???). These are iridium plugs. I'm beginning to wonder if the left LTFT is usually in a lean condition (-11 to -13%) because there's a little too much fuel coming through from #5? It's odd because both STFTs roughly average around zero (except when accelerating). Surely the odd O2 signals wouldn't have this much effect? I would've thought so as they are usually close enough.

I forgot to mention this engine had a full rebuild about 10k ago, but it's still stock.