Hi 1994 LS400
At 90k miles, you should have your timing belt & water pump replaced.(dealer here charge about $1,300.00) Did you have that done already?
Also you should have your spark plugs replced at 60k miles.The next spark plug replacement is 120k.,
Also it is good to replaced your engine coolant, transmiision, diffenertial. Did you have your brakes fluid replaced recently.
What about air filter? How old is your battery?
Don't forget to check your power steering reservoir because LS400 has a history of power steering leakage. My 91 LS has 68k miles but I'm in my third power steering assembly
that's all I can think of right now