[Barnsnake]

Something to think about......
If the coolant is slowed down so it spends more time in the radiator cooling off, it also spends more time in the engine heating up.

A radiator can extract a given amount of calories (or BTUs if you prefer) per minute, assuming a constant differential between the coolant and ambient air temperatures, and a constant air flow through the radiator. Changing the velocity of the coolant through the radiator will not change the amount of heat extracted.

A very slow coolant velocity would result in cooler radiator exit temperatures, but very high coolant temperatures exiting the engine. The slow flow also keeps the coolant in the very hot cylinder head areas for a longer time, creating the risk of steam pockets.

A higher coolant velocity keeps the engine temperatures more consistent, front-to-back and top-to-bottom. A higher velocity is increasingly superior right up to the point where pump cavitation occurs, or extreme turbulence disrupts the flow. Such problems would only manifest themselves at high engine RPM.

In short.... If the engine is overheatng at low speed, reducing the coolant flow will make it worse, if anything.

As noted above, the first thing to determine is the actual temperature of the coolant exiting the engine. Don't beat your brains out trying to solve a problem that doesn't really exist. An infrared thermometer reading a non-glossy area near the thermostat and temperature sender is a pretty good test. Assuming there is no air in the system and the coolant exiting the engine is well above the thermostat temperature, you need more air through the radiator, or more radiator.