The OBDII scanner I used to check for stored DTCs belonged to my Son. Its a generic low cost scanner and may not even report anything other than the mandatory OBDII emission codes and might have been incapable of reporting the DBW error codes. It certainly does not support logging. I do have a lap top based diagnostic tool that is designed specifically for Audi - VW products and supports logging. The vendor says it is OBDII compliant; but, will not guarantee that it will report anything other than the mandatory OBDII codes on non VAG vehicles. The tool has a vehicle limited license and if I enter the NSX's VIN in the license and the logging function does not work I have wasted that license provision. I think I may have to investigate one of those OBDII dongles if I can find one with internal memory for logging. This erratic operation has so far been very infrequent (or I just don't notice it most of the time) so I would have to leave the data logging function continuously active and hope to capture an event. The OBDII device needs internal memory for logging because driving around all summer with a laptop attached waiting for something to happen isn't really an option - another reason why I did not use the VAG diagnostic tool. After the last occurrence of this where it was obvious that there was a definite step change down in speed lasting about 2 seconds and then it restored and then repeated, I think the changes will be slow enough to be captured in the OBDII data stream. Out of curiosity, do you know how many of the measured and calculated parameters are available through the OBDII data stream. Is something like the accelerator position sensor voltage accessible?

Because of the definite step change in speed lasting about 2 seconds and having it repeat with the same pattern a few times, I am currently inclined to believe that this is not a misfire of any kind. Misfire tends to have a random component to it and this behaviour was, in my opinion far too periodic to be misfire.

In the absence of any obvious causes I think that data logging through the OBDII port may be the best option. Now I just need to find the appropriate tool.