Joe,
First, welcome to DOF!
Second, when the light is blinking, it's time to stop......quickly. Steady light is usually a minor problem. Blinking light is usually something that is going to cause damage pretty quickly.
I'm assuming a gas engine here.....
But from the sounds of it, you're getting detonation and the knock sensor is tripping that light. I say this be cause you said it's idling fine, but under load it's hesitating.
More than likely the ECM is trying to retard the timing to keep the knock in check and it's no longer within spec or it's not able to retard it enough to prevent the knock, tripping that light.
Could just be a be a bad batch of gas, but more than likely it needs a tune-up.
Has the van had a tune-up any time recently?