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Were the GM coolant tablets used? It would be unusual for the pump to be the problem.
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I f you feel there is no, or reduced flow then I would change the pump to be safe.
If the system was flushed the tablets should be used. They go in the lower radiator hose and dissolve and seal imperfections in the castings of the heads and block. But in your case you might try a product called Blue Devil, I have had tremendous success in cases even worse than yours. It is a high end block and head gasket sealer. Most parts stores carry it. Make sure you follow the instructions and chances are extremely good it will fix your issue permanently or at least for a very long time. They advertise a 85% success rate in Northstar engines. Here is a link to the manufacturer if you want to read more about it.
I believe the pump is a possible cause, but it is unusual and head gasket and casting flaws are more common. These issues are common on these engines and it is rarely the water pump.
When it was flushed, did the radiator seem to flow well?
Follow the instructions exactly for the best results, add it just like adding coolant, make very sure you leave enough room for it after adding some coolant first.
It is a fairly difficult job and the book calls for 16.4 hours of labor.
It is not something you want to do without a lift.
I think we discussed that early on or I at least mentioned it might be a blockage.
One possibility is reduced radiator capacity. Over time the radiator can collect sediment that cannot be flushed out. To check this get the engine up to normal temp and turn it off and then being careful of the fans which can still turn on, compare the temp at the bottom of the radiator to the temp at the top of the radiator, if it is much colder at the bottom than the top, or there are any areas that are cold, that is a sign it is partially plugged and the capacity is reduced.
If available you can use a non-contact thermometer.