I'm flumoxed

[flyboyjack]

Hey Folks,

it's been a long time for me,.... lost my '71 C10 Cheyenne Super in an accident and sorta quit for a while. (Should have stayed "quit").
I bought a '63 C10 long bed stepside from a collector/dealer of sorts. If he could have, he'd have lied about how much air pressure was in the tires.
Anyway, I'm now in the throes of trying my best to get this "steed" back to reliability and roadability. Both are a challenge and expensive.
Here's my dilemna: Living in southwest Florida, the outside temps as in most of the USA are sky high. Suddenly with the A/C on, the temps immediately go to 220 and stay there. At idle and even driving at 45-60 would not bring the temps down. As soon as I turned off the A/C temps came back to 190.
So I changed out the fan. Installed a Flex-a-Lite 3000cfm and cut 6 3" holes into the shroud with "flappers". This helped the temps, but the puzzling thing is that at road speeds 45-70 MPH, the temps slide up to 210-215 from 190 (A/C is on). Then when back in stop and go driving, the temps fall back to 190 or so, (A/C still on).
I currently have merely a single core. Trying one fix at a time. It seems the new fan gives me better temps, but I cannot understand why highway driving raises the temps and stop and go brings the temps back to "Normal"). Obviously there are no hills,.. it is flat Florida

Anyone got any thoughts/help?

Thanks.
Jack