I have been reading through this thread and wanted to correct some of the information and provide some of the solutions that have worked for my FE in trying to resolve this same problem.
Note: Your problem as has been mentioned appears to be air flow at lower speeds. Your fan & shroud appear to be fine at 3600CFM and the diameter does not seem to be an issue at 16". I would suggest checking or lowering your fan relay settings, using a manual switch when you get into traffic and reducing your T-stat to allow more cooling margin and possibly adding pushers to help at lower "city" speeds.
I started to write my findings and then I found the article that applys and provides the tech data (though it was written for Jeeps it did a better job than me).
cooling article
here's another article on "coolant"
coolant article
Bottom line that worked for me:
-25% "coolant" mixture (note that water is best for disapating heat but it has a lower boiling point which can be raised under pressure (pressure cap) and with the addition of some "coolant" below 50%
-Shrouded fan (I have a Blackmagic 16" 2800CFM) note that CFM is more important than fan diameter
-Manual Fan switch (used in traffic conditions since the "automatic" Blackmagic temp control is a contact type bulb on core and does not come on until the temp is at 200 which doesn't switch on the fan early enough to provide fast cooling
-Pusher fans. These provided an extra 5deg cooling as well as faster cooling
Edelbrock waterpump- the CNC design does provide better balanced cooling in an FE, less cavitation (turbulance at impeller) and did reduce operating temps 10deg.
160 T-Stat- contrary to the article I've found that a 160 provides an extra operating margin for cooling with no downside, I did add a single 7/32 hole.
13lb Rad cap- I've seen folks using 15lb+ but had some concerns about that much pressure and my older expansion tank (see below)
Results are that since making all of these updates to my cooling system my car (FE 500hp) runs at 170 highway and 180city and on hotter 90deg days in city in stopped traffic for 20+ minutes it may get to 195deg. I've been running these updates for 15K miles without any cooling issues......except for a recent pinhole that developed in my expansion tank due to age
good luck,
Mike
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