You need larger pots right away. Not only the top of the plant, but the roots, need cooler temperatures.
Outdoors, I HIGHLY recommend pots that are slightly larger than you would think you need, and spray-painted white to reflect the sun's energy and not transmit it to the root zone.

My mother told me when I was little that the Clematis likes its head in the sun and its feet in the shade. Apply that to your plant.