That was my second guess [8D]. I'd say the fan shroud isn't large enough. Not enough air is being directed towards the radiator. But of course I could be wrong.

 

is there a way to adjust the shroud or does that mean i need a different one?

 

Well, you could move the fan in/out with a different fan spacer, but you'd need a shroud that is set up for that diameter fan.

 

I'll put in a couple thoughts...The shroud does look shallow, maybe by about an inch or 1 1/2". I don't think that fan can go much farther forward than it is without hitting the radiator. The fan pulls air thru the radiator towards the motor, it doesn't push it the other way. My fan looks similar to that and has the same issue. I have tried diff fans and shrouds and am now thinking of going to an electric fan.

You're problem though, is that not enough air is moving through the radiator at idle.

 

I had exactly the same problem and it took me years to fix it. It is definitely an air flow problem. I ended up getting rid of the fan and shroud and fitted a spal electric fan. Now I can sit in traffic for hours with the air con on in hot weather and the car doesn't move over 190-195 when the fan comes on. That makes driving the car a lot more fun when you don't have to worry about overheating

 

I agree its related to air flow which is why temp goes up in traffic and cools down when you start moving. But I have an almost identical shroud, stock sized 3 corecopper/brass radand stock fan. My car gets hot in traffic on a hot day, but doesnt overheat. I get enough air over the rad because the passages are clean (rodded last summer here).

what i am saying is i dont think you should rule out a partially blocked rad either judging by a bit of rusty looking stain around the top of the cap there.

how old is your cap and is it a proper "recovery" cap for your o/flow system?

if you do decide to go electric, speak to derwin. i remember him having all sorts of issues with overstated cfm from electric fans!

 

I live in arizona and have a 3 row new radiator, the right shrould for a 6 popper and a flex fan and the temps go bonkers during the summer ( 110 degrees and a/c on ) droped a electric fan in front of condenser/radiator ,,manual switch setup and problem solved,, something to consider