Brandnew car repaired without a diagnosis

Bought a new C200 in February 2015. Failed to start on 2 ocassions in may 2015 and towed to the dealership. Diagnosis not determined and no faults identified as the car was working again on both ocassions. Pleaded with the dealership to replace the car since it has presented a problem and they can't find what was wrong. Instead they replaced major electronic components of the car which makes very uncomfortable. I tried to cancel the contract with the financier but the dealership refused to give consent. I refused to collect the car from the dealership and still paying for it. I even rented a car for 3 weeks because they demanded back their courtesy car that I was using. I had no choice but to buy myself a new small car to use in the meantime.