Replaced leaking H2O pump, thermostat, bled air out. previous owner was a coolant additive jockey .It overheats at slow speed and very little heat entering the car.

What'* that mean?

I bet it means that the previous owner used many coolant additives in the hope of winning the race to selling the car to the next poor sap while still driving it . . .

. . . and if so, that probably means they were trying to fix an expensive problem the cheap way, possibly creating another expensive problem in the process.

I suspect the radiator and the heater core are plugged solid with radiator additives.
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How many miles on the car?

How many miles have you owned it?

Do the radiator fans work?

What engine does this car have?

He added stop leak to compensate for fluid leak and loss, motor seems fine, daughter seems to think it glows in the dark because previous owner worked at nuke plant, fans do seem to run less consistent than they should. Motor runs strong except when impeded by overheating governing acceleration. Thank you for inquiry, Timber Jeff.

A radiator will likely fix your over heat. The heater core for heat.
Was the water pump the issue for the stop leak? Where was it leaking from?

I don't think these have the ability to govern themselves for heat . . . or are you saying that the driver elects to slow down because of the overheat condition?