Here's a shot in the dark. Check you're exhaust system for a 1) catalytic converter whose insides are falling apart and/or 2) a loose clamp or broken pipe along the exhaust system. Here's my reasoning ... When the cat goes, the screen pieces may block the exhaust and give the bog down condition. While addressing this on my 740 I also found that corrosion of other exhaust components had allowed everything to vibrate so that the wires to the oxygen sensor were loose. This explained (I think) the intermittent lean mixture code I was getting that was stumping my mechanic. Anyway, it was the bogging down and the associated rattling that led me down this diagnostic path. It's easy enough to check out once the weather warms up.

Todd