I had something similar happen to me - in my case everything would be fine, then when the AC kicks in the RPMs would drop, the AC wouldn't engage and it would die with a CEL code indicating some MAF issue. I looked up the CEL in the VX workshop manual (you should download the free copy of the CD if you haven't gotten it already) and it indicated that the MAF wasn't returning a signal within the 5 or 6 second window, which forced the CEL and put the truck into limp mode. This in turn prevented the AC compresser from engaging as it requires a minimum tach (and the AC keeps trying, which dips the tach each time and eventually killed the idle).

So using the manual, I went down all the troubleshooting procedure (which is also basically what your dealer/mechanic did to troubleshoot). I ended up going through every process without finding a fix - each led to a different item and each was testing using an OHM or AMP meter to ensure the signals were within tollerances. So I finally just took apart the entire intake and using gasket material, re-made a new gasket attaching the throttle body to the plenum (mine had a small tear fixed with permatex - I figured it might have been enough to cause a leak, which may have been causing the MAF issue - that was my thinking, anyway). While I was putting everything back together, a small boot leading from the harness to the front of the block just fell off - turns out there's a temp sensor there that wasn't fully clipped - I guess it had worked itself off and because it's covered by a rubber boot, you can't tell by look or even by feel without pulling on the wire - it even felt like it was on by hand and when the truck was first started, it made contact but the vibration of the truck eventually caused a break in the connection. This temp sensor was what was causing my issue - once firmly reattached the CEL didn't come back.

I'm not saying that you are having the same issue - but I think it might be heat related. I've found that when it's really hot my VX becomes very sluggish, even with the AC off. With the sensor feeding back wrong information, everything at the intake adjusts the mix and when it wasn't getting the right reading, it dumped the CEL and went limp. Nothing in the manuals would have led me to this as I guess it's outside the realm of usual issues. With it being so hot and since you didn't experience the same issue up north, I think you should take a look at the sensor and wire leading to it - it's right under the throttle body.

Oh and for those of you wondering why my TB is back in the front, I removed the SC plenum and replaced it with the old to have my drive rebuilt and still be able to use my daily driver.

-- John