Problem with BT Broadband

I use a speedtouch 330 modem and my broadband line just got upgraded to 8Meg. Most of the time, whenever the speed crosses 5.87 Meg the modem looses sync. I just installed the new drivers released by Thomson for this modem. The ISP (BT Broadband) is unaware of any such modem issue. They already sent me a replacement modem but still the problem remains intact. There is no line fault.

Can anybody help me? I tried everything in my disposal.