What is the setup on the Kentrox? Is it a CSU/DSU? Make sure youre channels = 128k as far as your AOR of the WAN goes. The ISP, hereafter refered to as 'They', have given you a default gateway for your router. Use wan killer from solarwinds to saturate the link both ways to see what kind of BW they are giving you. If it's close to 128k, use wan killer to test the LAN, watch you interface counters on the Cisco to how many Packet per Sec. you are getting.

Check the usual suspects. DNS response time, large temp internet files. My guess is that 'they' are oversubscribing BW, or they have a very low burst rate on the link, does your connection improve during off peak hours?