Why does my 98 deville continue to overheat?

Run the car without the thermostad to see if it still overheats. If so check the heat sensor it might be faulty.

check belt tension for the water pump if belt too loose pump will not send enough water in and will cause your engine to overheat, also check that your coolin fan is working,

What do you think is overheating? 220 is not overheating.
Is the engine cooling fan coming on?
Any debris (dirt, leaves, bugs, birds, etc.) stuck in the radiator or A/C condensor? (the space between the condensor and radiator is like a belly button, its a space that collects a lot of stuff).
Also it is never a bad idea to change the Thermostat.

do the heater core coolant supply hoses both get hot at the firewall in the engine compartment? if yes then you have a defective temperature control blend door actuater motor.

Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

Check the radiator, hoses, and water pump for leaks. If you don't see any coolant loss, the problem is most likely a faulty thermostat. Hope this helped and best wishes.

did you try replacing the radiator cap and the temp sensor?

As an added thought. With the overheating documented as GM Elec posted. Have the serpentine belt and especially the belt tensioner checked. Tensioners will fool you sometimes.

i remember trying this. if your water pump is faulty. while the engine is off, spin the radiator blade. if it spins, then your pump is out, if it stops immediately, (it has a cluth of sorts) it's ok. also use a lower degree thermostat, like 160 degrees, then it will open sooner.

check for air bubbles in the system also check the pressure cap and have it tested also squeeze the upper radiator hose together and if it doesn't open up in your hand right away the water pump could be weak work on getting the air bubbles out first before going to the water pump