@compaq_12 first I would make sure which package you have, I believe comcast now has different speed pricing.
2ndly, call comcast and tell them that you are getting intermittent speed, and that the lines keep cutting out, they should send a service tech and they should be able to fix the speed. They had to put a booster in the basement where I used to live, to improve the signal. Are you also getting their cable service? if so, then for sure, they should be able to improve your speed.