Re: Shower issue..
As stated by roker, check with your neighbours about their shower problems. If yr water is very hard, you might consider getting a water softener - this will prolong the life of your kettle, washer and dishwasher too. If the shower is replaced the same thing is going to happen again anyway and to be fair you can't expect the builder to keep replacing shower units forever, (although I agree 8 months for a fully operational shower isn't very long)
Perhaps the builder is cheesed off with the all shower problems and your complaint just came at the wrong time - I'd try approaching him again.