I have a symptom that I'd like to try and diagnose with TunerPro and hopefully someone on the forum can provide some help. My mechanic sat in the vehicle while it happened, but his only diagnosis was to hook it up to a scan tool, which shows everything is good.

The symptom, is what feels like an occasional miss under load. It's more frequent at altitude, though has started to happen at sea level. It is quite bad at altitude sometimes, to the point that at even freeway speeds, you can feel the van deaccellerate slighlty. I will be towing a large trailer over mountains this summer and I would like to fix this before hand.

I've discovered that it coincides with a brief rise in spark advance, by viewing the graphs in tuner pro. Here's an example.
Attachment 8880
Just after 8:36, it happens (a slight increase in spark advance, coincides w/ the stumble), again at 8:44, then 8:51.

I also found a couple bigger strange reactions in the spark advance, shown in these images.
Attachment 8881Attachment 8882

Any ideas on what could be causing this?
If anyone would like the saved full files from tuner pro, I can upload those as well.

Thanks for any help.