I'm hoping I don't need a new shower unit here so any ideas or how to fix the following problem would be appreciated.

I bought the house last June and everytime I turned the electric shower on the lights in the house would dim slightly until I turned the shower back on again. Normal I think due to the high current its using?

But last week my girlfriend said she noticed that the light on the shower cord unit (bit attached to the ceiling) would blink and the water would go cold for a second before the light came back on. I could live with that, but its got progressively worse. Now its...

Pull cord to supply power to the shower
I turn on shower
Light turns off immediatley (shower has no power, so cold water)
I turn off shower
Pull cord (off)
Pull cord (on)
I turn on shower
Light turns off after 0-30 seconds
Repeat

Its doing my head in :-( Any ideas?

Edit: When I pull the cord the 'power light' can stay on for hours. Its only when I draw current that it goes off.