TC and Airport Express = Airport Express very very slow to almost dead

when I wireless connect my Powerbook to my TC internet is normal fast.
when I wireless connect my Powerbook to my Airport Express (in the other room) which is setup to extend my network internet is very very slow to almost dead. streming music from powerbook to Airport Express has a lot of drop outs.

this setup was perfectly working with airport extreme (1G) and Airport Express.

what could be wrong?