I 'used' to have an AOL account, until I encountered the same problem you describe. AOL continously side stepped me by blaming BT and telling me to persist with them over bad lines. One thing they never owned up to, was that when I 1st got the account, I was furnished with 5 numbers to dial out to, this was 'upgraded' to 4 and then 3. Inevitibley, it became harder and harder to dial up and stay online.
It was finally admitted to me, after obtaining some information from a BT engineer, by AOL, that they have reduced the access numbers to save costs, even though this causes oversubscription.
I changed immediately to Tiscali and haven't look back ever since, in fact I went on to sign up with BroadBand with them.