When in very hot weather driving slow air conditioning blows hot air. Any ideas?

Insufficient air flow into engine bay.

...which would lead me to suggest having a look at your radiator / condenser fan operation. Still, with low engine speeds, your compressor isn't turning fast enough either. Optimal air conditioning compressor operation occurs somewhere above 1,500 RPM -- if you're crawling along, it might be a bit to slow for optimal performance.