Why would my sonata Lunge when braking and then lug until it stalls when stopped completely?

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. You did not mention how many miles are on this Sonata, and if there are any warning lights or other indicators illuminated. It sounds like perhaps there are two concerns; one related to a very high engine speed at idle, which is likely due to an unmetered air leak or some kind of control valve sticking high. Testing must be performed. The second concern is possibly with the transmission itself, with the lugging and performance. Testing is needed separately here also. A shop near you can get into it; see the links below. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.