On my VPS, I was having attacks so I installed CSF firewall which did a great job. However on a few of my sites, specifically proxy ones, every second or third page you visit will be a 403 Forbidden error. After about 20-30 seconds, you can refresh and it goes away. I suspect CSF is causing this, because it just started to happen after I installed it. Is it thinking there are too many connections or too much bandwidth and its blocking me or other users just using the proxy? Is there a way to make it slightly more tolerant?
You might want to start by looking at your apache error_log and looking for the 403 requests. If you don't see any, they may be remote errors, if not, then you will want to check permissions/ownership/htaccess on that directory.