Hi,
If it happens only sometimes, it means that there is something different with the product in the order where it doesn't work.
It might be the product code, the product name or the price.
I would first recommend to compare the products between the orders where it work and where it doesn't.
If you can't find it, the best would be to place an order in the checkout and when you get the redirection message to authorize.net, stop the redirection with your browser, and look at the hidden HTML form near the redirection button. It will contain all the data sent to authorize.net. Then, click on the button. If you get the error, then, the HTML data will be valuable in understanding what Authorize.net doesn't like about it.