If you are overheating when moving slow and the fans are running (or should be) the problem is something to do with the fans. Some things to check include:
The circuit breaker for the fans is popping
Check that the fans are both working
Check that both fans are secured to the motor shaft
Check that both fans are pulling in air from the front and blowing it out the back
The otterstat switch could be bad
Verify the otterstat switch is on the bottom of the pipe (early cars had it on the top)