You have a few options:
a) fix whatever performance problems you've got on your site that are slowing down the acknowledgement to authorize.net upon return. https://www.zen-cart.com/tutorials/i...hp?article=402
b) fix whatever might be wrong in your SIM settings https://www.zen-cart.com/tutorials/i...hp?article=403
c) change to using the less troublesome and more advanced AIM module instead: https://www.zen-cart.com/tutorials/i...hp?article=373


BTW v1.3.9 has been released, and contains many updates, including improvements to the authnet modules, as well as general overal system security and much more.