Recently had similar problem with my 2000 LS430 here in Perth (235k). Had both Tappet cover gaskets replaced (leaking badly) along with PCV hose which was perished due to age, also replaced Platinum plugs. Cost exactly $628.90. Parts and labour GST incl.
Couple of weeks later temp gauge went off the dial, called out RAC - top radiator tank split???
No way it had anything to do with rocker cover gasket replacement - radiator just decided to die without warning, no previous coolant loss, replaced radiator and car has been fine ever since.
Probably not much help to you I know but maybe have rad. pressure tested to see if any leaks under pressure.