ok after reading all this i have a few comments to make:
1. above mentions testing voltage at battery etc .... my suggestion is see what the voltage is at the charge wire on the alternator first, once you know what the alternator is putting out then go through the rest of the connections and see if you can find drops, for example my gemini has a vr v6 alternator, at idle its putting out about 14 - 14.4v but with the battery in the boot im loosing nearly .5 v to the back of the car, so you really need to know what the alternator is putting out so you can see where and how much its dropping.
2. before you go and buy yourself a you beautis fuel pressure gauge there is a simple test you can do, run the car, let it heat soak or whatever so you get the 18:1 afr then unplug the fuel pressure regulator vaccuum hose from the manifold and this should give you somewhere around 10-15 psi increase in pressure, if that makes the wideband straight away go from lean to rich you have found your issue, weather its a regulator or fuel pump not handing it will be up to you to work out from there
