aldan :
wheres the rest of the story.make,model,engine,etc? what does your engine oil look like,milky?been a mechanic for over thirty years but still need all the details to make any kind of diagnosis.any sludge in the rad itself?
I have a 97 ford F150 4.6 litre with a blown head gasket mine looks more like Clumpy old milk and its on the oil cap when you unscrew it and you can see it in the radiator reservoir
unless the oil itself is milky it more likely you do a lot of short trips without a complete warmup of the engine.if the oil looks like coffee with cream then i would look at the head gasket,although these dont have a history of these problems.