i must say i finally do not believe all the fancy rumours.
i think they had somekind of an initially undetected smoldering fire in the electronic bay which disabled one system after another - starting with acars. after system failures began they decided to turn back to malaysia , using the heading mode .
just in the turn the fire melted through the structure resulting in a rapid decompression. the crew oxygen bottles, stored in the electronic bay, failed and the pilots were out of order. the decompression by itself also put off this fire.
the autopilot continued to work and stucked in the turn on a heading towards indian ocean where the plane continued until fuel exhaustion and then crashed.
any news on altitude changes etc are just false rumour from the malysian side.
it maybe that "simple".