My van's A/C turns off when I start speeding up?

Sounds like the ac is low on freon. The system has safeguards built-in that if the low-line pressure drops too low or the high-pressure line gets too high, the compressor will be cut-out. In this case it sounds like the low pressure line is is dropping too low. That usually means that their is not enough freon in the system. The fact that it does work at all tells me that it may only need a light topping-off. Top it off with a combination mixture of freon/sealer/conditioner. Be sure your radiator system is right, because a great working ac will add additional heat to it, -extra heat that you may not be seeing now.