Vehicle Shudder problems of the 2000 Mitsubishi Diamante

While traveling between 55-70mph and trying to pass another vehicle transmission would go into neutral position, pressing on accelerater would cause it to skip to 3rd gear, sometimes hitting gear downshift real hard, vehicle shakes, would hit neutral first before going into correct gear, had taken vehicle to dealer, vehicle was test driven, and nothing found placed on diagnostic, no problem codes appeared. Transmission goes into neutral instead of third and goes into neutral when down shifting, this has almost caused a head on collision, consumer also states while going downhill, he shifted into third gear and vehicle went faster.