Hi, I had a Rover 216 from new and I had a similar problem which was not resolved in its warranty period and it persisted for some years. The plug leads were identified as being suspect by a couple of garages which got the vehicle back on the road until the next bout of wet weather. Under the warranty the Agents took it in over night and carried out an inspection the next morning at a cost of £25 and found nothing. Eventually I sorted it myself by renewing the HT coil assembly which on the Rover was at the back of the engine low down. I was not an easy task but the car was never a problem again for over ten years. Incidently spraying with WD 40 is a quick fix in some instances but on plug leads it acts like fly paper and particles will stick to the sprayed surface until there comes a time when the current flow will escape to earth by tracking through the layer when enough current carrying particles collect. When this happens you will find the leads will not wipe clean too easily, just a thought. Good luck.