This all suggests too much go juice to me.
Weeping injector maybe?
Like I said, it would be nice to see a scope trace of the injector pulses, then it would be obvious if the ECU were commanding too much.
Checking the plugs might highlight if it's one cylinder or all.
By the way, my tailpipe was black very soon after fitting but I don't think it's running particularly rich. It still has a bit of a flat spot in the middle of the range but fine at set off. Almost opposite of what you have really.
Only other thing I can think of is temp sender giving bogus info to the ECU although I think it's common with the temp gauge (but not certain)
I'm pretty sure, something is putting it at the wrong place on the fuel map when setting off and dumping too much fuel in there.