This has been an interesting thread (I just stumbled on it waiting for decaf to brew). Two things:
First, I think everyone is missing the reason temps spike when there is air in a system. It isn't the sending unit that sees the air (and "heat"); it's an air pocket at the thermostat, which keeps the thermostat from opening when it should. Thermostat coils don't operate in steam, and there needs to be a huge spike in temp for a "dry" thermostat to open. Once it does, coolant circulates and the temp plummets fast. (This is why I also drill a 1/8" hole in all my thermostats)
Second, I NEVER run a fresh cooling fill with the cap on, until it is about to geyser. Fill a fresh system, turn the heat on full, start the car with the cap off, and stand ready at the radiator with a gallon of coolant (to fill as the level drops when air is replaced with coolant), a rag, and the cap. Looking in the fill neck, you will easily see the progression - stagnant cold coolant while the thermostat is closed (perhaps a trickle of circulation if you have a 1/8" hole in your thermostat); levels that drop along with bubbles that appear as air is replaced with coolant (top off to keep the levels correct, as needed); faster visible flow and mixing of coolant as the thermostat opens; finally, level rising to the cap height indicating full circulation has been reached and temp is approaching full. At this point I will let it circulate until I see zero bubbles (constant bubbles indicate a head gasket leak), let it overflow just enough to make a bit of a mess, then I'll cap it.
This method, along with a hole in the thermostat, has kept me from having air pocket issues for as long as I've owned cars. An 1/8" hole isn't enough to cause operating temperature differences, all it does is allow coolant pressure to push the air through the hole, so the thermostat isn't "dry" after a fresh fill.
To the issue at hand for this thread, it seems to me to be a blockage of some sort in the radiator, if the problem is persisting. For my money, I would:
1. drill a hole in the new thermostat and install it
2. drain the system
3. remove the lower radiator hose from the radiator
4. put a garden hose in the radiator filler neck (cap off), turn it on about halfway, and allow it to flush the radiator. don't use full pressure, just a steady flow. It should exit the lower hose fitting as quickly as you are putting it in the top. If it doesn't...you have a blockage. I know - it's a "quality part". Even quality parts have defects sometimes. If you do see a blockage or flow issue, remove the radiator from the car, turn it upside down, and do the opposite - put the hose in the lower radiator hose fitting and let the water flow out the cap opening, as a back-flush to try and dislodge whatever is stuck. *note: make sure neither your radiator, nor your water pump, are notated as being "reverse-flow" units.
4a. if you want to rule them out, perform the hose-flow-test on the other two members of your cooling system- the engine itself (remove the thermostat, hose in the thermostat housing, water flows out the lower hose), and the heater core (connect the hose to your heater feed hose, disconnect the return hose, and monitor those flow capabilities as well - again, NOT at full pressure!). You have now flow-tested all three sections of your system - radiator, engine, heater - as well as all the connecting hoses.
5. Once component flow has been proven, reassemble everything (with a hole in the thermostat), fill the system, turn on the heat, start the engine, and run/bleed it as noted above.
If you do all these steps, including 4a, and you still have air pockets that appear after a proper bleeding, and you don't see any external fluid leaks...you have a compression issue somewhere, either a cracked block or a blown headgasket.
