94 camry overheating

My 94 Camry drives fine for about half an hour with the temp needle in the middle of the gauge, then without warning, it will start overheating and I’ll have to turn on defrost to cool it back down. I already changed the thermostat and the fan seems to be working fine. Also the car seems a bit slow to accelerate until it starts overheating, then it accelerates just fine. Any thoughts?

Your symptoms are arguing against each other. Verify the fans are turning when it overheats. If you haven't yet, I would start with a backflush. You can buy a backflush kit just about anywhere. Use a system cleaner to get any crud out.

Look into the coolant temp sensor circuit. It not only sends data to your gauge but also in a primary sensor in fuel injection/spark advance management. The fact you can disapate heat with the defroster argues against a bad sensor but, I would not rule it out. If you're getting a check engine light the sensor will generate a code 22. The sensor can be bad without throwing a code or light. They cost around $20. Heres how you can test it:http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/13/e2/83/0900823d8013e283.jsp

I replaced the Radiator and the thermistat, now it no longer over heats.
The thing is though that now the acceleration is sluggish in first gear.:banghead:
If it's not one thing it's another.

PS. sorry for not posting sooner. i almost had my finger torn off by a print press, this is the first chance i had to get back to work on the car.