As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLT AT FL 380 IN SMOOTH AIR, .78 MACH AUTOPILOT "A" DISENGAGED FOR NO APPARENT REASON. ALL OTHER INDICATIONS WERE NORMAL. AUTOPILOT "A" WAS RE-ENGAGED SUCCESSFULLY. GROUNDED-YES. REPLACED MCP, REF B737-800 AMM 22-11-34-400-801. INSTALLATION AND LLM, ALL TESTS PASS.