Here's my theory,
Your thermostat is bad and stuck open.
When you test it in the shop, the temperature is controlled by the electric fan which doesn't come on until around 205 degrees.
When you drive down the road, air is forced through the radiator core and the engine temp goes down because the thermostat is open.
If you changed to a lower temp thermostat like a 160 degree one, that will do the same thing.
I don't understand why the engine temp would show OK when this happens. It should be reading too low.
Also if the coolant gets too low in the system, the water pump will not pump anymore.
Check for bubbles in the coolant that would indicate a leaking head gasket. If it leaks enough, it could blow coolant out of the system.
Also check the coolant reservoir hose for plugging. Remove the hose from near the cap and blow through it, you should see bubbles in the tank.
I've seen them plug but only in dirty conditions. Then the tank will fail to replensih the sysetem.
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer
[This message has been edited by davidpozzi (edited 12-06-2000).]