Sorry for the long delay, this started in November.
In summary my street car starts/runs fine if started daily but if left for a week or so is was very hard to start and runs rough for 10-15 minutes or a 3 or 4 mile drive.
After checking everything in the fuel system, vacuum leaks etc. I changed the distributor cap after some advice here - problem solved!! no issues at any time. Thanks.
I did check the old distributor cap very closely with a magnifying glass and could not find any cracks etc. It was also interesting to see that my old cap had no vent but the new one had a vent cap system - both Mopar parts. So I suspect, as mentioned here, that moisture built up in the cap and with some mechanical issue (crack, dust etc.) caused tracking inside the cap, after the small drive the moisture was removed and back to normal!