When you changed the thermostat did you also replace the rear thermostat seal? the thermostat is designed to either recirculate the fluid through the block when cold or redirect it through the radiator when hot. this is done by either closing the outlet to the radiator and opening up the bypass port when cold, or closing the bypass and opening up the outlet to the radiator when hot. the thermostat has a rear seal at the bypass port to ensure it is sealed off when the thermostat is open to the radiator. if this seal is bad the water will continue to bypass the radiator and shortcut back to the engine. Item 33. part number 928-106-163-00.
Another possibility is the mechanically driven fan. over time the silicone fluid can leak out of the fan clutch and then the fan doesn't drive at full speed even when the clutch is hot. if the fan rotates very freely by hand (with engine off of course) then it is likely that the fluid has leaked out.. another way to tell is to have someone watch the fan when the warm engine is shut down. the fan should stop fairly quickly (about half a revolution as a ROUGH estimate). If this is the case, the fan clutch either needs to be replaced or have silicone fluid added (
Here's a place to start. search rennlist if you need more info).