Could the EGR system partially clogged with soot also adversely affect the engine's ability to compensate for altitude? Or is that likely a whole separate problem? And can I just take it in to my mechanic and say "please clean out the soot and other glunk out of the EGR system"? It sounds like that would probably be advisable anyway after 313K miles. Also, do you think it might be a sticking wastegate, after all the turbo is all or nothing, it's not like it degrades. Maybe the heat is causing the wastegate to somehow stick open.

Thanks for the info on the wastegate conversion. Looks like a pretty complicated conversion, but maybe there's some more info out there now on the necessary bolt-on parts.

I've always hated excessive electronics on cars. Wish I still had my old Chevy Luv with the aftermarket Weber carb and NO emissions controls. I could do a full tuneup on that truck in about an hour flat and get that thing singing. Ran perfectly with nary a miss and I never had to bring it in for any service I couldn't do myself in over 10 years of owning it. Driving up to the mountains in New Mexico from the flatlands of Texas, altitude compensation was done by pulling over to the side of the road when the engine would start running a little rough and manually leaning the carb. But I digress....

Thanks for the help.