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Low coolant, bad thermostat, clogged radiator, bad water pump, in that order. A missing shroud will give you problems and if it only happens when going slow a bad fan clutch.
I had your same problem when I used some cheap "Radiator No-Leak". It clogged up my thermostat. I took out the stock one (192 degrees) and stuck in a 180 degree thermostat. Now she runs quite cool. I'd highly recommend the 180 thermostat regardless of what you do.
Just look in the Haynes guide for your ploblems and woes.
Painless test for the fan clutch is to take a piece of heater hose and with the engine running see if you an stop and hold the fan. If you can bad clutch. I like direct drive and flexi fans my self. The brown is rust so do a flush and refill. Theres a two part flush that's actually citric acid that works best but be sure to neutralize it and do a goood clear water flush.
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