It never made sense to me that they'd have crashed into a peak on Sulawesi if they were overflying for Manado at Flight Level 350.
My guess is that they EITHER took a large drink of heavy precipitation and lost both engines (just like’s happened a number of times before with the 732) OR Alternatively that they had an upset as they were very very near “coffin corner” for a 737-400 (i.e. close to the high altitude stalling speed at FL350). That’s a very high cruising altitude for a loaded 737-400 less than halfway along their 2 hour route of flight. There could also have been some orographic wave turbulence over the mountains of Sulawesi (depending upon wind direction at the various levels).
If so, the aircraft may have stalled and spun in sudden severe turbulence and may now be somewhere on land/in the ocean – having broken up during recovery efforts at high speeds/g on the way down. This latter guess gets my confident vote for a more likely cause. At FL350 the precipitation shouldn’t be as heavy as at lower/medium levels. However turbulence in and around thunderstorms, even without the orographic influence of high terrain, can be severe. That area has had lots of very nasty ITCZ related weather just lately (ferry sinking etc).
