Cadillac Repair: overheating cadillac sts, cadillac sts, coolant system

Question
QUESTION: Hi my 99 cadillac sts drives fine but all of a sudden the temp jumps up and it starts to overheat, but if i put in neutral and give it some gas it returns to normal. It sometimes returns to normal when I start to drive again after sitting at light or stop.
also when i checked the coolante reservoir i found some rusty flakes in it. Do I have a blockage or is it something worse.

ANSWER: Are the cooling fans running ?.

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QUESTION: yeah, is their anything else it could be? also i want to thank you for your response. I greatly appreciate your help:!

Answer
I would flush out the system with a flushing agent that you can purchase at most any auto parts store and then purchase the sealant pellets recommended for your type vehicles coolant system and add the coolant to the radiator.

If the vehicle continues to overheat you have the very very common of combustion chamber pressure getting into your coolant system via faulty head gaskets caused by stripped threads of the head bolts.