The earlier LS400, ie, the MK1 & 2, not sure about the MK3 had only two coil packs, one for each bank, the most likely one failing is the passenger side due to the heat. A failed coil pack knocks out four cylinders and causes serious overheating in the exhaust system. This is something I discovered with my MK2, the only time it let me down in five years! The good news, only two coil packs and only £25 each (at the time)