1 - Recently, I have been experiencing significant clunking in the trannie/rear end between 3rd and 4th gear in automatic mode, in low RPM situations. It is difficult to simulate (intermittent), but happens when driving slow and in low RPM's, and accelerating. Anyone else experiencing this with 70K+ miles?

2 - I am thinking of going to Pirelli Scorpion "ATR's" instead of Scorpion "Zero" (average 25K on Pirelli Zero, 18" 255/55). Feel free to suggest if you have experience with Pirelli Scorpion ATR or better yet, tires with 50K warranty that ride smooth and track well. All comments appreciated in advance.

I have had no mechanical issues with this car and in fact I love driving it. No rattles, no problems. Handles well in snow (Tahoe) and is a pleasure to roll on freeways. Great car!

 

Not sure about tranny problems. I have almost 41K on my Cayenne and no such problems yet?!?
Regarding tires, I use Michelin Latitude Tour HP tires, same size as you. I don't care what anyone tells you, you will not find a true high performance tire that will give you 50K miles. The truth about tires is that those that are stickier, softer, and handle the best, wear faster. That being said, I suspect that these tires will last to about 25K, as they are wearing better than the Pirellis that came stock with it. My Goodyears on my Ford GT lasted to about 6K miles (with a few track days), my wifes Bridgestone Potenzas on her 350Z lasted to about 20K miles, the Pirellis on the Honda S2000 were gone at 20K as well. I fine the same to be true with my bicycles....softer rubber and better grip come with a price...one that I think is worth it. But everyone has to decide this for themselves.