slow ppp connection

I have a redhat linux 6.2 running kernel 2.2.14 and I have a very slow ppp connection.
My ISP used chap (MD5) and I am able to connect to the ISP successfully but after I connect if I ping the isp the packets return after 20,000 ms approx. but somereturn after 150 ms followed by a period of no activity. I am confused to what might be the reason for this. I have checked the interrupts and there is no interrupt clash. I feel that there should be some kernel parameter which should be set or there should be some parameter that should be passed to the pppd.