Noticeable changes in performance?

[Dpeters]

This is certainly subjective, but I've recently noticed that my "butt dyno" shows a lack of power while driving down the road, but a significant increase in power is shown after making a stop and getting back in the car. I'm not sure if this is due to the 370 doing some switch to performance mode (vague documentation about the 350Z doing ECU changes to the air flow, etc for performance mode comes to mind) or if it's my imagination. Does anyone else experience this?

I suppose it could be a bad batch of gas or something, but it just seems odd.

Just to hedge the questions before they come, the following are clarifications:

1. No A/C or heater was in use.
2. Same tank of gas and at the same level.
3. Same time of day, temp, driving conditions, etc.
4. No passenger drop off for weight differences.

This was simply driving to work when lack of power was noticed... stopped at gas station to pick up a drink (no gas fill) and when I got back in and turned the car on, power seemed to come back. This happens more than this once, I'm just using this as an example.