i had that problem kind of but i was loosing fluid it wasnt goin in the motor or going out but my temp would fluctuate and when i sprayed on my car at the track all the fluid disapered most people told me it was my head gasket that was blown but i still havent figured it out but good luck though

 

its been running fine, i check my temp more than my speedo

 

verdict- air pocket and not really chunks of dirt, but kinda.

 

ok, do this. this will check for blockage and bleed the hell outta the system.

jack the front of the car up
open radiator cap
turn engine on
turn defroster on high heat
allow it to warm up

air bubbles should come out little by little, keep filling as necessary...also, check that the fluid is circulating. you should be able to see the anitfreeze flowing from left to right when the thermo opens. when engine does get to normal operating temp, feel upper and lower radiator hoses to ensure that there is no blockages (this in addition to looking into the radiator).

replace radiator cap
lower car and drive

if it does the same again, time for leakdown test...you may have a BHG.

know this, the gauges in the s chassis suck ass. the temperature is a 3 position switch.....below normal, normal, overheating (nuclear style).

i would say the sensor is a problem...but that was until you stated you are filling the resevoir.