That sounds like more a line problem than a driver problem.  Especially since you are getting the wide connect speeds. 

If you have a cordless voice phone in the residence, disconnect it from the telephone line and see if it make any difference - corless phones are known to cause connection and speed problems.

Actually, when you listen to the telephone line with a voice phone, do you hear any static or "cross talke" (voice from another telephone line)? if you do, you need to call your phone company. 

Other items that can affect the line including answering machines or even a voice phone, or the inter nal (In house) wiring, the telco wall jack or even the cable that connects the modem to the phone line.  It will take a little trouibleshooting to try and isolate the problem. 

As far as drivers, most Dell supplied hardware is proprietary and can only use Dell provided drivers.  But, I seriously doubt that your problem is drivers.  It's a classic telephone line problem (either the line, or in house wiring and the phones).

If you have call waiting on your phone service, that could be the culprit.

It happened with me. When an incoming call was made, it would dissconnect the internet.