di-614+ and mn-720 dropped packets and so on

Hi

To be brief, I have a network @ 11mbps AP - dlink di-614+
and mn-720 in 802.11b mode. I have some problems, as
downloading files from lan connected computer is really
slow and pinging that machine shows different response
times (2-200 ms!) and 25% of packets get lost. It's that
way even when I'm near the AP! I dont know what to do.
Also, my MN-720 card won't recognize net at channels 1-6,
only 8-11 are valid. What the...? do I have to return
this card?