Toyota Repair: Overheating, toyota 22r engine, radiator cap

Question
Ted,
My 94 Toyota 22R engine is overheating. I can see no puddles under the vehicle but it losses fluid. I've checked the oil to make sure the fluid isn't leeaking into the crankcase but its clear of water, vice versa, the radiator has no oil. I've changed the thermostat as well.
Left to idle from a cold start it idles at 20,000 rpm, as it warms up it idles at 10,000 rpm. Then suddenly, it begins to gradually rise 15,000 rpm. When it reaches this point it begins to rev up and down between 15,000 and 20,000 rpm this goes on for about 2-3 minutes then I can hear a small boom or knock, the overflow tank begin to spew steam and you can see steam trying to come out of the radiator cap. Then the idle normalizes to 10,000 rpm. After awhile it starts all over again. It's almost like pressure is building up in the cooling then released all at once instead of gradually. I believe my fluid is disappearing from the overheating and being released as steam. Therefore no puddles. I've exhausted all my ideas of what is causing this, has this ever happened to you, and what did you do to fix it?
My apologizes for the lenght of this.
Greg  

Answer
Hello Greg, welcome to my expert page. The first thing that comes to mind is that there could be a headgasket problem, so before you spend any more time or money I suggest that you have the engine checked for this, most shops can do a chemical block test, this checks for any hydrocarbons in the cooling system, so try to eliminate the headgasket as the source of the problem.