Thermostat or No Thermostat?

SLC Duster Dude said
360 .40 over, stock pistons, Eddie heads, Air Gap manifold, medium Eddie cam, headers aluminum radiator and electric fan.

I always have an overheating problem when the temps get over 85°. Not driving down the highway, but as soon as I slow down or wait at a light. The radiator is in excellent shape, no crud or anything. New 13psi cap too. I took the thermostat out last year thinking it would help but it doesn't. The electric fan is 16" high flow (2400 cfm) and goes almost top to bottom but not side to side.

Thoughts? Ideas?

I know you have a lot of suggestions already, but I think your main problem is not getting airflow over the whole core like a regular fan with a shroud does.
When you are on the hiway you do get airflow over the whole thing, so even though your engine builds more heat under hiway speeds it's obviously capable of cooling it with flow over the full radiator core.
Betcha if you did a clutch fan with a shroud you would stop having heat issues.