The single engine rated pilot and 2 passengers were returning from Florida after picking up a newly acquired Cessna 402. The pilot called Macon FSS for fuel availability, and began a night, VFR descent. At about 5300 feet MSL, the airplane began rapid altitude excursions, while in a tight, right hand turn. During this time, it descended 2600 feet in 37 sec, then climbed 1200 feet in 12 sec. Subsequently, the airplane impacted gradually rising terrain at high speed. The engines were buried, and all prop blades showed evidence of power at impact. Weather in the area was reported to be: sky partially obscured with fog. An investigation revealed that the pilot was not multi-engine rated. A pilot for the seller flew with the accident pilot a few hrs prior to the crash; he reported that the accident pilot had difficulty flying the airplane. He recommended that the accident pilot get additional training prior to leaving Florida, but the accident pilot refused.