OK, so the problem has to be:
x_line_item=1<|><|><|>1<|>25<|>
and in the authorize.net documentation I can see that this should be:
ID<|>Name<|>Description<|>Quantity<|>Price<|>Taxable
So you are missing a name and description for your item in the cart... perhaps they are necessary. That is the only potential problem I can see with item 1.