Well there is a known fault on the car which is water ingress to the spark plugs orifice. The remedy for this is to check that the coil and plug holes are clear of water, then use silicon grease to waterproof the coil and HT cables.

If not this when would ask if the mechanics used a up to date diagnostic equipment such as Auto Logic or Launch, as the fault should come up as a fault code.

Failing this then the most likely fault could be the ECT (engine coolant temp) sensor, but as stated previously would have thought that the diagnostic check would have identified this.