comcast HSI download speed fluctuations

[IsHnU]

i am wondering if this a normal because it really gives me no speed benefit if i can only download at top speed for few secs and then drop by the 100's back down. back home i have dsl with stable 200kb/s download which is sometimes faster at downloading than with the comcast fluctuations. i chose the setting with 1mbit and port is open.

i do happen to have the linksys wrt54g2 V1....