No one has problably replied because what you are describing shouldn't be happening. You are correct, if the switch port is in a particular VLAN then it will be sgemented at layer 2 from all other VLAN ports.
I'd first double check the VLAN config as Smurf suggested.
What DHCP server(s) are you using? is it a router or a Win/linux box?. If the later, then make sure the DHCP server for VLAN8 is connected to a VLAN8 port on the switch. If your using a router as your DHCP server (i.e router on a stick for both VLANs), then I'd double check the masks set for the two scopes/pools (apparently /24 in your case).
Still no hope? Then what happens when you set one of those ports to VLAN8, does the PC connected get a VLAN8 IP or a VLAN1 IP?