I had something similar a year ago. Car started fine, ran for a few minutes, then would run like a pig, conk out, and not start again for about 20 mins, then it would run fine. This all happened after the summer, and a period of very wet weather. I removed the distrib cap (which was replace recently) and it was in horrible shape and very wet. There is a rubber seal in there which was not making contact and sealing properly. I got some RTV and effected a quick repair and the car runs nicely ever since. However next time the distributor has to come off it'll be a bit of a mess, and I have to find some proper rubber to use as a sealant.