99 grand am overheating v6 3.4L

1999 Pontiac Grand Am SE
V6 3.4 L
170000 miles

Original problem, car overheated and service soon light.
Service engine soon light directed me to misfire cylinder 1, replaced spark plug and corrected service engine soon light.

Now the overheating issue, replaced thermostat, no heat, and car overheats.
Flushed heater core both directions with garden. No heat until thermostat opens. Initial start car overheats and thermostat does not open. After warming car up and shuting off the thermostat finally opens. Its acts like once the thermostat opens it is fine but the first startup the car overheats.

Bleed cooling system Did get some air out. .Blew air thru bypass line and another line back to coolant reservoir to check for blockages, no blockage found.

Last night once thermostat open went on 20mile drive car acted fine thermostat seem to open when it should car did not over heat. The only thing I notice the vents would blow hot air and then cycle to cold and then back to hot.

Sat car for about .5hour topped off coolant fluid and initial start it overheated, thermostat did not open.

How does this bypass from top of water pump play in all of this?
Does this sound like an air lock?
Should I flush radiator?
Should I drill holes in thermostat?

Thanks for any help in advance.

I'm worried that you have a faulty water pump.

I would advice against the hole drilling.

Also check to be sure that you don't have bubbles. If your engine overheated it could have cracked a gasket.

The hot/cold is usually blockage, air trap or faulty water pump.

Sounds like air trapped in the cooling system. Most likely trapped in the heater core loop.

Though this might not explain why it overheated originally.
Was it low on coolant, or was the thermostat the cause..?

You could check your water pump. Easy to remove and a fairly cheap part.