Car overheats only at certain times of the day

[shift7]

I just picked up an 88 Turbo. Awesome care, I love it and couldn't be happier. I live in Phoenix, AZ and the car overheats when I drive home during the hottest part of the day. In the morning in runs completely fine, and right now I just took it out for a little 5 mile blast pushing it incredibly hard and it didn't show any signs of overheating at all.

Just to help out, I'll run you through my scenarios:

Morning: Drive to work (about 20 miles), normal pace, around 90 - 95 outside - No problems.

Afternoon: Drive home (20 miles), easy pace, try not to boost at all, 110-115 outside - overheats like crazy

Night time: Drove about 10 miles total, about 100-101 outside, no sun though, pushed it really hard - no overheating.

So, if anyone has any suggestions they could give me as to where to look I would be very thankful!