ADS-HELL problems

Hi !

got a client with old 11G adsl router, which complained about disconnections of wifi and of internet. Replaced her with Tp-link 8951ND router TWICE, But it didn't preformed well - internet wise - and froze while trying to access it's WEB UI.
Talked to support - told me to replace to 8961ND model.
Changed the router, But then a wired computer responds slowly to the internet and on another router it responds well.
Bought a used router that was sold by her ISP D-link 2650U, And it works better, but now it cant recognize on one of the wired computers a networked HP printer, And she complains that wifi works slow on her ipad and her laptop pc.....
After 4 routers I don't really know what to do. I thought :
1. install the 8961ND again and replace the network card in the wired PC that worked slow with this router
2. Turn her 2650U Adsl router to modem and add another router as the dialer
Any other suggestions ?

Thank you

Update :
In the end I tested the client's network with the 2650U and it works ok.
She also got Dlink 6740 router which supports 802.11N, adsl2+ AND VDSL, So I configured it too so the 2650U will function as a backup router.
So now basically the new 6740 is the main router and everything works, And in case of a problem with that router - She can just connect the 2650U and it will restore her network.

Update all the drivers and firmware if you can.

I think the client's house is haunted :)
Everything worked well for a week, Now her hubby cant send emails through his ipad/iphone while connected to the wifi.....He CAN send through 3G or through his windows 7 laptop.......Going there tomorrow 7am with all my ghostbusters tools (oh and a router too :) )

Log into the router, and look at the logs. Hopefully, you turned on logging.

Reinstall drivers and check settings. It should be enough I think.

Well, I installed a different router and it solved, but eventually the problem was because the iphone/ipad used SSL and it shouldn't use it with that provider....It was strange since it worked WITH SSL on the second router and the ISP support said it can't be......