car fine in stop & go traffic?

This car is fine in stop and go traffic going about 40 mph, but from stop to 80, it will overheat (and spout fluid all out of the reservoir).
The thermostat and water pump have recently been replaced.
There is no coolant in the oil or vice-versa.
It's an 88 Taurus 3.0L V6. Anyone have any suggestions? TIA
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[This followup was posted to alt.autos.ford and a copy was sent to the cited author.]
nospamme+ snipped-for-privacy@gmx.net says...

Does the cooling fan (or fans) work? Is there debris blocking the radiator? In particular, junk might be inbetween the AC condenser and the main radiator.
I would guess it can just barely handle the heat. When you accelerate up to 80 (!), you generate a lot of heat over a longer timeframe.
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