Cannot send email to AOL

For months, we have had intermittent issues sending email to AOL addresses. I have worked with AOL engineers and verified that our DNS (MX records) are correct. We have RDNS and spf record and AOL has verified that they are correct. I have established feedback loops all to no avail. When I contacted AOL yesterday, they said that our IP was blocked for spam. I asked for details so I could investigate as the feedback loop provided no input. NADA. One day I can send to AOL and the next day I cannot. We are having NO issues with any other ISP. Any suggestions?