Perhaps look at it this way... (and forgive me if I'm remembering tank size and other stuff wrong but I think I remember 7G) To properly fish-in cycle something that small (anything under 10G is basically tiny in the tank world and tends to exaggerate a lot of things) you'd choose perhaps the equivalent of 2 tiny fish (that's IF, I'm being theoretical here.)
You have all the factors working against you (well, most of them, lol).. the tank, being small, will allow concentrations to swing much faster (it would take 10 times longer to raise a concentration the same amount in a 70G.) The biomass of fish, being larger than the tiny 2 fish example, will have the ability to pump out concentrations more quickly, then, on top of that, remember that the A-Bacs triple the amount of nitrite as compared to ammonia, right? So, putting all 3 of those together, its not at all surprising that you could see sudden spikes in nitrite!
(I think you've just reduced fish (good!) and I didn't read back through the whole thread Sting, so forgive me for just being numerical and possibly not sensitive to how hard you're trying!) WD