OK, I did see a bulletin in the GM manual about polishing the neck of the surge tank. I will include it below. Have you tried it?
#00-06-02-001: Engine Runs Hot, Overheats, Loss of Coolant (Polish Filler Neck/Replace Radiator Cap) - (Jan 5, 2000)
Some customers may comment on one or more of the following conditions:
The radiator filler neck may have an imperfection in the sealing surface.
Important: DO NOT REPLACE THE RADIATOR.
Using a piece of 400 grit wet/dry sandpaper backed with a flat piece of wood, polish the filler neck sealing surface using a circular motion.
Replace the radiator pressure cap with a cap of the same part number as shown in the GM Service Parts Catalog.
Ok, I was meaning the tricks on the Internet you have tried. I see the items you listed and they are what I would do.
When it does overheat, does it just show warm or does it push fluid out?
OK, I think you may have an issue in the radiator. Some sort of restriction that needs the higher rpm to overcome. I will post the chart from the manual, but it seems you have done most everything.
DEFINITION: The engine temperature lamp comes on and stays on, the temperature gauge shows hot or coolant overflows from the coolant recovery reservoir onto the ground while the engine is running.
Check for a missing or damaged radiator upper baffle and/or center air deflector.
Is the baffle or deflector missing or damaged?
Go to Step 8
Go to Step 2
Check for a loss of coolant.
Is there a loss of coolant?
Go to Step 3
Go to Step 4
Fill the system to the specified level. Refer to Loss of Coolant .
Does the engine overheat?
Check for low coolant protection. Refer to Coolant Concentration Testing .
Is the coolant to the correct correction?
Go to Step 5
Check for a loss of system pressure. Refer to Cooling System Leak Testing .
Is there a loss of system pressure?
Go to Step 6
Check for a faulty engine coolant temperature (ECT) sensor. Refer to DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage and DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage in Engine Controls - 3.5L.
Is the sensor operating properly?
Go to Step 7
Check for a cracked coolant recovery reservoir or a leaking hose. Refer to Loss of Coolant .
Is the reservoir cracked or is the hose leaking?
Repair or install new parts as necessary, then retest.
Go to Step 9
Check for incorrect drive belt tension. Refer to Drive Belt Replacement in Engine Mechanical - 3.5L.
Is the belt to the correct tension?
Go to Step 10
Correct for damaged water pump driveshaft.
Is the water pump driveshaft damaged or is the seal leaking?
Go to Step 11
Check for obstructed radiator air flow or bent radiator fins.
Is the radiator air flow obstructed?
Go to Step 12
Check for blocked cooling system passages.
Are the cooling system passages blocked?
Go to Step 13
Check for improper direction or inoperative cooling fans. Refer to Engine Coolant Fan Motor Replacement .
Are the cooling fans and the motors operating correctly?
Go to Step 14
Check the thermostat. Refer to Thermostat Diagnosis .
Is the thermostat stuck in the closed position?
Go to Step 15
Go to Step 16
Replace the thermostat. Refer to Engine Coolant Thermostat Replacement .
Check for faulty water pump. Refer to Water Pump Replacement .
Are the impeller blades broken or eroded?
Go to Step 17
Check the radiator cooling capacity.
Is the proper sized radiator being used on the vehicle?
Go to Step 18
Consult the current parts catalog. If necessary, replace the radiator. Refer to Radiator Replacement .
Is the repair complete?
There are no recalls. Also, there has only ever been a cluster replaced under warranty. Nothing else on the record.