Over heating 98 chevy cavalier

Sure if it clogged. Mix CLR with water, soak your rad and heater core with it then backflush them (you want to push the stuff out not force it further in). Under what conditions is it overheating though? Idle)slow speed or under acceleration?

The Cavalier has a special bleed procedure to get the air out of the cooling system. YouTube it.

It was when idling. After switching thermostat that stopped. But since then it's more when acceleration. The fan was also swapped as well. It's also not holding coolant either. Stop leak was used when I'm not sure.

Head gasket or cracked head. Losing coolant was a critical detail.

An obd reader was hooked up more than once. That never showed up. Will it show up when hooking and obd readet up?

A code reader won't tell you if the head gasket is blown or the head is cracked. There's no sensor for that.

Is it a physical leak or is it just disappearing? Can you smell it? When a vehicle temp rises while accelerating but starts to come back down while easing off the throttle and or idling is usually means you're low on coolant, it doesnt take much. IF the thermostat has a jiggly little pin or a hole in the body it is meant to be facing upwards so any trapped air can slowly escape. Check coolant everytime engine is cool, top up as necessary and if it wont fill up and stay there after a few days you do have a more serious problem as Jeff stated.