I had the same problem. In my flat, I dont share the ADSL with anyone, still I had the same porblem

atlast purchased , LINKSYS - ADLS modem. There was some special rates like QR.180 and I grabbed. Now it is fine,,, too good connection.

when complaiened to QTEL, they advised me to come to Villagio QTEL outlet and purchase QTEL modem, which I refused and purchased the LINKSYS one