Nissan: Rough Idling and flat spot. V6

Nissan Maxima 1999 3.0 litre V6 Saloon S.
(In Japan this model is named 'Cefiro')
I have had this car completely topped tuned, new plugs
etc.etc. It still idles rough every now and again.

I have used injector cleaner additive to the petrol.
This seems to help, but only for a short time.
I think it improves when I fill the tank with petrol,
but that may be only coincidence.

Besides the motor idling rough I sometimes experience a
'flat-spot' in motor reaction when I dramatically try
to increase speed coming onto a freeway.

Any ideas out there what it could be?

note: I have been unable to take the car to a Nissan
service outlet as getting a suitable appointment this
close to xmas is causing some problems.

TIA

Rob