2004 Nissan Sentra Overheating

I bought a 2004 Sentra w/83,000 miles not too long ago at an auction. I drove it off the lot and the Temp. Gauge went to the Hot level after driving it for about 10 mins. for about 15-20 seconds then it goes back down to half level. It does this all the time.Took it to the dealer to get a diagnostic. They told me it was the Thermostat, Radiator and head gasket might be leaking. Charging me $1,300 to replace head gasket, $400 for radiator and $300 for Thermostat. I changed the radiator and thermostat myself (New Hoses too), cost me about $100 for both parts. However the Temp gauge continues to do the same. Is it true the head gasket might me leaking and needs to be replaced to avoid this overheating problem?