93 explorer over heating

my 93 explorer overheated. the gauge stayed by the L in normal and i never let it get into the hot, but when we pulled over the coolant was boiling over and there was no radiator fluid in the radiator we let it cool down and the radiator was still empty but the over flow was holding all of the fluid! Im fine as log as im over 35 but if i am at a stop light it will shoot right back up. does it sound like a fan cluch or a thermostat issue. i just had the radiator flushed about 3,000 miles ago and there are no leaks. dobbs wants 180$ to "TRY" a thermostat. I dont have the money to experiment. please help!

never mind. it was the thermostat! i paid 170$ for a 6$ part! the diagnosis was as expensive as the labor so they just put it in anyway. DONT let this happen to you! replace your thermostat every 15k. its worth the trouble and the part is only 6$!