Every time when i fly, mid way my plane turns off lnav and vmav and turns left and goses in a nose dive. The fmc reports when after i stable the aircraft and try to return on AP, " NOT ON INTERCEPT HEADING" Ive done so much flying in the 737, but overall new to the max era. It will miss a turn somewhere then start to go down. I know my fmc is right, had it verify by some people. Any ideas?