I was having similar problemw ith my quality score dropping again and again , and i had to change the domain name , register a fresh one...and so on..
Here is waht i did ,
I took had a site with full of articles , about year and year and a half old , with some decent rankings in google , created a landing page , removed the navigation on this particular page , and used it to run the campaign...and voila...I had a QS of 9 , afterwards.
I realised , That google might be giving preference to old sites in adwords as they are old and have gained certain authority in google after a while...and these are not exactly 5 pages sites as well...
Each page was for one keyword and one keyword only and had onsite
seo done properly..
Cant show you the url , as i am still using it to advertise on adwords...
If this is something you dont want to try ,try to add moer pages , this will definitely dilute your %of leads...but it will be good in the eyes of google , and will not ban your account.
I hope you find this useful
Nix
HIt Thanks if you find this helpful.