hi,
there are a few possible reasons - firstly I would phone your ISP and get them to do a line test
one reason would be congestion on the network, but there are plenty of others - how long has this been happening (ever since you had BB or just recently), has it coincided with anything else eg central heating timer going on.
Is your modem near any other electro magnetic devices (EMI) that could be causing interference?
Do you use a wired or wireless connection?
For starters, I would do a FULL power cycle - switch off your PC, then unplug yout modem(complete power off), leave for approx 5mins, switch your pc back on, wait until desktop is fully loaded, then reconnect your modem, give it a few mins then try again.
A xXx