Was just about to post the same issue. Climbed to FL380, went to Auto Cruise, and then the aircraft seemed to lose altitude slightly. After that all hell broke loose and the AP was essentially useless. If it was a tail strike then PMDG did an amazing job simulating an explosive decompression and non-catastrophic loss of a the rear pressure bulkhead. This was on an AAL flight from KMIA to KLAX.
Tried an AAL flight today from KMIA to SAEZ and climbed to my designated cruise altitude of FL350, engaged Auto Cruise and stepped out for a bio break and coffee refill. When I returned the aircraft was at sea level wobbling around stalled with Wheel Well Fire annunciation on the MFD and my brake rotors glowing red hot. I assumed it was my fault however for running the center tank dry while on Auto Cruise.