Hey guys, I have a question re: the 350 LM1 motor i have in my '77 Z28. I redid the motor a few years ago, mild upgrade over stock, but one thing has always remainded constant with this motor, even when it was in 'bone-stock' trim. It could idle all day long in hot humind summer heat and have no temp. issue, but as soon as it cruises for awhile, at about 2,700 - 3,000 rpms, the temp. heads into the 200-210 range. As soon as i slow down and come to a stop, the temp. starts to head more toward 160-180, even with the reduced water flow at low rpms.

The thermostat is a 180 degrees, the radiator seems fine, although the original one and the water pump is new.

What could be causing this?

Radiator or air flow. Engine generates more heat while cruising, either the rad is plugged, good possibility since it's original. Or your fan is not pulling enough air, not as likely. Cruising should force enough air through the rad regardless of fan or shroud. Could also be water pump. I'd still lean towards rad, has it ever been flushed?

Check your Clutch Fan, I had the same thing happen with a 71 Monte Carlo. The fan was stopping when driving down the road and go up to 230 and would work OK when your just sitting at a light back down to 190.
I put a new Clutch on and it was fixed.
Bring the motor up to around 3000 and hold it there and see if the fan slows down or comes to a stop like mine did.