is my key faulty ?

Take your car to the people who supplied the new key. Tell them its been giving you trouble '0ff and on' ever since you had the new key.
Your new key must have come with some kind of warrantee and if you have the receipt or credit card statement you can invoke the arguement that the key is not fit for the purpose intended. i.e. to start the car every time as is reasonably expected.
The problem with this type of intermittent fault is they never replicate themselves on demand, but that is no excuse for the company to demand a payment up front without any recompense if they find that the sender unit in the key does have a fault.
It is encumbent upon them (within the warrantee period) to prove comprehensively there is no fault-then and only then are they entitled to seek payment for their labour.
Stand up for your rights- Ignition keys with coded chips are designed to work for 'life' -not a few months.
Have a read of this: www.moneysavingexpert.com/.../consumer-rights-refunds-exchange

GB in Wales
March 2013