Place sugar, corn syrup and lemon juice in a medium saucepan and heat on medium-high just until sugar is dissolved.
Add grapes and bring mixture to a boil.
Reduce heat to medium-high and let boil for about 4 minutes, or until grapes start to release some liquid.
Reduce heat, and simmer for another 4-5 minutes, until grapes start to break apart and they are completely softened.
Let mixture cool in the refrigerator for at least 30 minutes.
Pour entire mixture into the bowl of a food processor fitted with the chopping blade, and process until grapes are fairly smooth.
Pour mixture into a large lidded measuring cup or bowl with a lid, and refrigerate at least 3 hours or overnight.
Pour cold grape mixture into the bowl of an ice cream maker and churn for approximately 30 minutes or until light and fluffy in texture.
Transfer sorbet into a lidded container and freeze for about 4 hours.
Sorbet is best enjoyed the day it's made. It can be frozen for longer, but let it sit out and thaw until soft enough to scoop.