i got the same problem as yours after my alternator is broken and i change to a new one. because the alternator is hard to take out. so i remove my cooling pipe. then i got the same problem as yours. i have 2 advices for you.
the first one is ask a lexus dealer to check your alternator is stock or a china-made one, it may depends. the second is to check your cooling system for air-con, maybe it is lack of pressure