Driving from Cluny to Sarlat area: mapquest says approx. 5 hours. Goes through Clermont-Ferrand, and involves some major roads. But are there issues with minor roads?
The minor roads down that way are slow. Use the 'E' roads, you'll be going 75+ just to keep in the flow. Good rest plazas along the way. You'll bypass Clermont if you pick up the E-70 about twenty miles north of there. The slowest part will be the last fifteen miles or so into Sarlat. Disregard mapquest; the trip will take about four hours, even with a quick stop.