How to target slow e-mails?

I have Small Business Server 4.0 running Exchange 5.0 for our internal and external mail. We have our ISP (Verio) hosting our domain and use ETRN to call our mail down to our server. For the last month and a half, about 20% fo our emails are coming in 3 days late, and no one can help us find a reason. The emails that are slow are come from various sources, at various times, some have attachments and some don’t. As the kicker, a customer can send us an email that we won’t get, send the exactemail again, we get it immediatly, then the next day we get the original. I need a way to find what is holding the emails up. Does anyone have any ideas?