It's hard to tell for me, are you still having coolant leaks? I would start checking stuff in the following order:
Check the hoses and radiator for any leaks. Look underneath the car when it's hot to see if there's any cooland dripping or in your garage.
Check for coolant on the engine oil, the 3800s have a tendency to leak from the lower intake manifold gasket into the cillinders.
Check the driver compartment carpet and see if its wet or if it smells like coolant, the heater core may be leaky too.
Change the TSTAT for a 185°F it's cheap and easy to do and also change the radiator cap, that's also easy. Mine wasn't working and I was throwing away coolant all the time.
Purge the system with the purge valve locaeted in the TSAT housing to get rid of air bubbles in the system.
If the coolant looks like mud, you also want to flush it and put new coolant in it.
Also check if both fans are working when it's hot. If not, you have to check the fuses. relays and temp sensors related to them.
