Some ideas:

Uninstall any module that you are not using ...

If using more than 1 payment module and sort orders number them in 10s to avoid conflicts ...

AuthorizeNet AIM is the better choice between SIM and AIM ...

If using AuthorizeNet, be sure to test can you do a transaction directly on the AuthorizeNet site?

AuthorizeNet requires a secure URL and cURL on the server ...