Was driving on the freeway at 85 on cruise control, all of a sudden it dropped off cruise control and decelerated itself very quickly down to 64/65 with my foot on the accelerator pedal the whole time. Extremely dangerous if there were cars behind me, I quickly switched lanes to the right lane. Could not accelerate it at all, when there were slight uphill, it would decelerate itself even if I'm trying to accelerate it. When there was slight downhill then it would pick up to around 74/75. Drove about 45 minutes on the freeway at 64/65, got to the city area, was afraid that it wouldn't accelerate if I had to stop at a red light. It didn't stall but I couldn't accelerate it properly at the red light, it would only go up to 15/20 with my foot on the accelerator. I just dropped it off at a Hyundai dealership today, they said they couldn't even look at it for another 2-3 days. Just hoping they would honor the 10yr/100,000 mile warranty. This could've been a major accident if there were cars close behind me.