After reviewing all the brilliant and eloquent technical input, it seems Jim's implied question (about what might have changed in his situation to cause the symptoms noted) remains unanswered.

Some related considerations:
1. Most of us here in Colorado on the east side of the Rockies (and that includes Jim) fly our jets at flying sites whose altitude is anywhere between 5,200 and 6,800 MSL. So, there is a significant difference in pressure altitude given that virtually all ECU parameters are typically established and validated at MSL. Jim did not mention if there was an ECU firmware update involved when he got the P-120 back. Being curious, I would want some insight into what ECU parameters were changed (if any) and some discussion with Bob Wilcox might be helpful.
2. The P-120s seem to be happy in any environment but at our altitude, we've found that not all the smaller turbines (i.e., the P-60 variety) are "happy" when fitted with FOD screens and, in fact, do exhibit the symptoms of a classic "compressor stall." Operating the P-60 without the FOD screen might prove insightful.

My 2¢ worth.

Mike