runnig hot on hot days

[Monster]

Lately I've been chasing a water tempreture problem and so I thought I would post to get some opinions.
I know that there have been a number of posts on the subject but I here's the details.

I have :
-FE406, iron heads, performance cam
-5.5gal cooling system with a 4.5" 4core custom brass radiator.(probably should have gone Alum. but didn't because this was a special fit made by Butler for the car)
-180 thermostat
-Black magic electric fan (set to come on at 190 however I have a manual override and have tried turning it on at 170 with no differance)
- 2"Steel tube upper & lower rad lines to short hoses to make the final connection.
-Timing is at 10BTDC
Had 50/50 coolant now 25% Water wetter w/distilled water (no change)
No air in the system filled it at an incline
3.54 gears W/top loader

The problem is that recently (this summer) the car runs at 190 to 205 on the highway@70mph on 80-90 degree days and on 90+ degree days it can get up to 215+. Around town it runs at 185-195. At night with high 70's temp it runs cool 170-185 in traffic and 160-175 highway. Last year even on hot days 90+ the temp stayed around 185- 190. With no changes to anything the car is now running about 15 degrees hotter on the highway and even hotter in traffic on those same days. I would suspect maybe a headgasket, obstruction, partial opening thermostat lean fuel but I run cool at night or on cooler days so it appears to me that it's something else


thoughts???

Thanks,
Mike Monster