Hmmm, I was afraid you'd say that... This makes the plugs less of a probability. Doesn't eliminate them, just makes them less likely the cause, although I still recommend going back to the factory specs. By the way, when you took the old plugs out, how did they look, anything especially pathological such as oil soaked, encrusted, any electrodes burned away?
The "linear" characteristic of this problem sure is baffling, suggesting not necessarily that the ECU has gone bad (again, possible I imagine) but that something's feeding it funny information. I'm really beyond my very limited automotive knowledge, but here's another thought, for what it's worth, the throttle position sensor