Fast connection to modem yet download speed only 50%

I just got a new DSL connection - 6 Mbps/512 Kbps with a new Motorola 2210 modem. Here's the problem: the connection coming in the modem is strong (8 Kbits/Sec 508 Kbits/Sec) but when I run speed tests on my comp, I only get a 3Mbps connection. If I move the network card to a different slot my connection runs at full speed, but then goes back to being slow after a couple of hours; however, the moment I move it to another slot it gets fast again. WTF?! I have two network cards: one attached to the mother board and one I'm moving around (D-Link DFE-530TX). The motherboard one gets the same slow speeds. Any suggestions other than get a new PC?

I'm using a Dell Dimension 2350 Windows XP |Celeron 2 GHz | 640 MB Ram | Broadcom 440x 10/100 integrated controller