Today while making a 2 hour road trip I played with speed and the cruise control.

It seems that when the engine is around 3k RPM's or better the cruise control stays accurate, through all sorts of terrain. (3k RPM=70+MPH)

I then slowed down to say 60MPH or 2500 rpm and then the speed was fluctuating bad again.

I went back to 3k rpm and all was smooth.

maybe this will lead to some answers.

You're right. It is BS. Having one, two, or 100 throttle bodies is not the cause of this issue. The ability of a cruise control to maintain speed within a certain band is related to the tuning parameters in the firmware for cruise control operation. An automotive cruise control uses a PID algorithm for control. The values selected for the P, I, and D parameters determine how tight the band is. Some manufacturers intentionally use "loose" values in the name of fuel economy. Other manufacturers use "tight" values in the name of comfort and/or performance. Sometimes manufacturers just miss it. For example, Buick issued a TSB several years ago to upgrade the cruise control algorithm on the Century to improve speed control accuracy. There may or may never be an update for the G. It all depends on how many people complain to Nissan and how they feel like responding.

Whats the best route of complaining to Nissan / Infiniti ? Complain at the dealership?

I believe that is it or file a compaint with the NTSB

http://www.ntsb.gov/

I doubt complaining to a government agency about it will do much good since it's not likely to become part of a mandatory recall campaign. If anything, it might become part of a future software update if Nissan determines it's bothering enough people, but that's not likely in my opinion.