One D-Link Router plus Two D-Link Wireless PC cards = INTERFERENCE???

Dear anybody,

I have a D-Link router running.
It has WEP encryption on.
It is dhcp-ing ip addresses out to people.

Two people in the room.
Both on ordinary cat4 ethernet connection - fine
One on ethernet, one using D-Link wireless PC card - fine
Both on wireless PC card - fine for a few moments, then one of the two
(or sometimes both) will get kicked off either the VPN they're
connected to, or the router itself!)

Now, when the person gets kicked off the VPN, sometimes its ok to
connect back on and carry on - still a hassle though.
Sometimes, when the person gets kicked off, they are actually kicked
off the router properly and an 'ipconfig/all' shows a completely
random ip address that the router's dhcp would not have given them.
Only action here is a reboot as ipconfig/renew (release) won't even
work!

Don't think its cordless phone interference as a one user on wireless
situation is fine and dandy (excellent sig strength and quality all
around the house).

So, question is - is it a fault that both people (or indeed more than
two) can't work with D-Link wireless cards?
If they should be able to, what's up with our setup?

Oh, and our kit is
D-Link DI-624 router
D-Link DWL-650+ wireless pc cards

Thank you very very very very much!!!!

Chiz.