Hi Buddy,
Good question,
Back connect proxies for me have always had a high fail rate they tend to be very slow, i have used the same services as you listed.
You have taken great steps to try and get over the problem I also increase the timeout limit.
I know it does not really answer your question but for me, i only use back connect for non-browser based bots as they work well.
Then for bots that require heavy lifting, i used dedicated.
Will keep an eye on the thread to get other peoples views :)
