Ever check the two ignition modules located down under the air cleaner box? Sometime they can have the electronics in them to fail for one or more sparkplugs. Had that happen with our LS. The modules are easy to check, and are not expensive, used on other cars such as Volvos.
Or, check for intake air leaks using maybe an unlit propane torch. Found a small air leak in the LS that way, a small air hose in the cruise control system was disconnected.