[V8] Mixture problem

[ville.hiivola at suomi24.fi]

I have a strange issue with the V8, i've been trying to sort out for a 
while now. Since when i got it, the spark plugs have been very white 
(running too lean?), even with new spark plugs. Also i have the 
infamous pulsating acceleration (not bad, but i can feel it there). I 
get code 00561 - mixture adaptation (add), not right after resetting 
codes but some time after it. I've replaced fpr and fuel pump and 
tested fuel pressure at idle, pressure seems ok.

Also i've tested all the injectors for leaking and flow, also 
everything looks fine. Many times i've tried to look for vacuum leaks 
by spraying crc or stuff like that, none found.

One thing i've taken note off is that o2 signal fluctuating quite 
slowly, even with the new o2 (about half crosscount per second on 
idle).

No exhaust leaks to effect the o2 sensor. Also i've checked rpm sensor 
and hall sensor signals by a self-made oscilloscope. Seemed to give 
nice signal on each rpm i tried.

I'm beginning to think do i have anything left to check.. Spark plug 
wires look good and have ok resistance. Distributors and caps look 
normal.

Could the ecu fail this way..?

One weird thing that happened this winter was that trip computer went 
dead and when i looked into it i noticed that the speed signal line was 
grounded somewhere. So i went on and rewired the speed signal from 
instrument cluster to every place it goes acording to the wiring chart. 
Now trip computer works again, but i wonder where actually the speed 
signal was grounded and would it have something to do with this problem 
also..? Perhaps wiring somewhere..?

Thanks for any help and sorry for the long post.. ;)