Here's a site I found quick one by google. I'm not sure if your limited to cable connections but here's a full rundown for Cable ISP's around Washington (I think)
I can't say much about the others but Earthlink and Comcast are names which ring a bell (Comcast is one of the ones I'm sure is good)
I'd recommend reading reviews / ratings of the ISP's if you consider changing for know problems etc
Wait for the outcome with Cox first however, it could just be a simple problem of they need to lower their transmissions speeds as your lines might not be able to handle it or could be faulty and need replacing.
Post here if you need further info I'll try and help. I'm not much of a specialist in IT just more of a jack of all trades. I don't know much about Cable internet I deal more with ADSL.
EDIT : Here's a better site : http://washington-dc.theispguide.com/ it gives youa full rating of the best ones (top-left)