To be honest with you we don't have your car infront of us, hence the reason why we're hesitant to give you a good analysis. If you live near Washington DC I could take a look at it. You could take the car to Toyota or Lexus for an analysis. Sure you'll be paying them close to $100, but they should analysis correctly. You could also have them fix it. If the problems isn't fixed, you can always until they fix it or refund your money.
BTW: Why did you pay $700 for the Coolant Reservoir? I though you'd change that yourself. I was hoping were going to pay $700 for the water pump, but not the reservoir.
I would suggest changing the thermostat yourself. It's really easy.
For your V8 Lexus SC400--I would recommend buying some 50/50 antifreeze from Walmart or Pepboys. I would also recommend a very large bucket, and a good set of Metric tools--no Standards. Definietely get some decent tools, specifically the ones below.
I picked up a set from Walmart for $17-18. Best investment. Also, you'll definitely need a new Thermostat. Lexus or Pepboys will have that. It doesn't hurt to get a good pair of socket wrenches either.
Lift the front end up on stands/jacks-bricks behind the rear tires. Drop on the floor, and search for the white coloured plug connected at the bottom of the radiator. Once located, put the bucket underneath and open the plug, and the radiator car. Let it empty out, completely.
You're likely to remove the plastic covers infront of the hose. There are two bolts holding the hose connector. Caution, there is a good chance some antifreeze will come out, so keep a towel nearby. Once you've removed the screws, move the hose connector out of the way.
The thermostat is inside the part connected to the ENgine. A counter-clockwise twist will remove that. Compare the two Thermostats to eachother.
Reconnect-clockwise, reconnect the hose back, and put the radiator plug back in. Fill up your radiator with the 50/50 antifreeze, and restart the engine. If you still have the problem, maybe it's time to have Toyota take a look at it.
Sorry I can't offer specifics, but like I said before, we don't have your car infront of us. You could always post something titled Help- with your city and state in the title. See if there's another lexus enthusiast who can analyze your problem.