Ford Repair: Brown Coolant..., white smoke, ford focus

Question
Thanks for the reply but now I got some new developments... Now when I drive on the highway for good 20-45min going about 65-75mph, I have noticed my water temp gauge slowly creeping up towards the red zone but then slowly goes back down to normal. This happens on and off. I haven't noticed this happening while driving off the highway such as on local roads. Now what's the car's problem?!!? Thanks.
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-----Question-----
I have a 01 Ford Focus and after getting my thermostat housing replaced and filled with green coolant, the coolant turned brown after a while. I had it flushed and then a week later same thing happened. Now yestarday I got another flush and a new thermostat installed. The mechanic put in the orange dex cool this time. I then check the coolant, and it's brown again, right after being it flushed. My overflow tank is stained brown, looks almost like gel susbstance. The mechanic told me there's still some stuff left in the block that he cannot possible get all of them out and the coolant will be brown but won't harm the car. My question is what the heck is wrong with my cooling system? Is something rusting out? My oil seems fine and it does not over heat at all. How can I tell if my water pump is going? Thanks.
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Tom-

  My guess is that it's just rust.  Where is it coming from?  Hard to say.  I'd put my money on the radiator, but that's just a guess.  While it's not a great sign, it's probably not going to cause any real harm.  There's a slight chance that it could be oil mixing with coolant (which would be very bad) but if that were the case you'd be noticing other things...like lots of white smoke on acceleration and other distinctly noticable things.  Hope this helps.

Steve

Answer
Tom-

  Hmmm...my guess would be that it's time to replace the radiator.  If your mechanic has done a flush and fill (which I sure hope he has) and you've still got something blocking up your system, my money's on a clogged radiator.  You might take the car into a radiator shop (yes, they have those) and have them test everything.  I'd also pull some of the spark plugs and check them for signs of coolant leaks.  I doubt you'll find any, but it's better to be safe than to be sorry and paying big $$$ for a new motor.  Hope this helps.

Steve