Why are you not using the same provider for both your sites? If your other processor does not have this problem then it would make sense to switch the one that is over to it.
Check with the payment gateway you are using to see if they have the redirect properly set up. It may be something in how it is set up. Is this a new account or an established one that just started having the problem. It appears that it is random and could be a browser issue.
There can be several things that it could be but you did not provide enough info to be able to pin point directly a solution.