This may be a dumb idea, but could it be something as simple as an accessory cycling on and off? You're not going to be using the A/C at this time of year in Maryland, but maybe there's something else going on in there? Perhaps something with a worn bearings taking extra power off the engine through the belt? I know this is a long shot, but I suggest it because my 740, (with a far less powerful engine), sometimes "surges" mildly, even with the A/C off. I was told that this is normal because the A/C compressor was not the only thing under there that runs in cycles, but I can't remember what else. (I could have been misinformed here). Does anybody else think something like that could do it?