Home page loads slow

I am living in Montreal, Quebec. My hosting servers are located in USA both Washington DC and Dallas. I get dynamic IPs which are usually like 66.x.x.x, 67.x.x.x or 69.x.x.x from my ISP. Every time when IP is like 67.x.x.x or 69.x.x.x, the first home page loads slow. It takes more than 30 seconds to load. After the first page loads up, all things will be running normally (4 seconds to 9 seconds). If I don’t clear the cache saved by browsers (Firefox or IE), I’ll load the first home page quick later on. However, once I clear the caches or wait for enough time like 4 hours (not sure the exact time range), the home page will be loading slowly again. Surprisingly I don’t have slow load issue with IP 66.x.x.x at all.

I’ve tried everything and communicated with both Hosting company and my ISP. But still no solution.

Anybody has an idea about this issue? Thanks for any inputs to help me figure out the issue.