As the airplane was vectored to avoid cells and areas of heavy precipitation, the controller queried the pilot about his inability to maintain assigned headings. The pilot reported that his autopilot had kicked off and that the winds are really weird up here. At about 1310, the airplane slowed to about 70 knots groundspeed on a northeasterly heading before it began an accelerating 90-degree right turn to the south. By 1313, the controller again asked, ...appears you've turned back to the northwest and...are you going to turn back eastbound? The pilot replied, I don't know what's going on up here. I'm working on instruments…acting really goofy here. Shortly thereafter, the airplane turned and descended from a northerly heading sharply to its right. The radar track tightened to the right as the target rapidly descended, then disappeared at about 1315 in an area that depicted heavy precipitation.