overheating/ eratic idling

so i drove my car to college the other morning (about 4 blocks) and when i get there i notice my temp is pretty hot. after school i fill up the rad just in case and start it up. it revs up to about 2500 and then shoots down to 12 and keeps doing this about once a second. so i head home and by the time i get home its almost in the red so i shut it off. i replaced the thermostat. still the same thing. i thought it might be a bad thermostat so idrove it around the block with no thermostat and still got hot. i took off the rad cap and let it idle till it got warm so i could see if the water pump was curculating. it still idled like [shizzle] and then got almost to the red and a big plume of steam came out of the rad and the temp went down and revs went down to 1200 then after a few seconds climbed back to 2500 and kept doing the same thing. what is wrong. any info would be appreciated. o and the head gasket is less than 6 months old but is cheap ebay brand. thanks