Ok, I want to try to understand...
Is the modem provided by your ISP? If so, they have told you that there is nothing wrong with it? How did they determine nothing was wrong? Did they trouble shot over the phone, technician on site?
If the modem is yours, then change the wireless channel, as it could be conflicting with other wireless connections in the house (camera, phone, security doppler, even older garage openers).
If you ADSL+ is establishing a connection, then dropping, then it is possibly wrong type modem, as your serving terminal may have been upgraded to DSLAM. Check with your ISP.
The speeds have nothing to with the dropping, unless you are perceiving a drop, when it is really just the bandwidth being consumed by 3 devices.
What clients drop? How do you know they drop?
Plese post back.