missfire

i have a 1991 sportline cabriolet…with a slight missfire…  i traced it to one of the HT leads which had necked off at the distributor cap… i decided to change the cap, rotor arm and leads for new…which i thought would solve the problem.. it seems the fault is with new HT leads that i have bought .. they are a good quality lead  with heat sheilds, however  i can here sparkig from around the distributor area…and i have a belt from one of the leads…i have checked everything over and it seems ok… i have even put 3 of the 4 old leads back on  leaving only one new lead on and tested with a voltmeter between the lead outer and earth.. there is a small voltage on 3 of the old leads  however  there is sparking coming from the new lead……am i overlooking anything…  i have tested thye coil and its ok…just wondered if anyone else had , had an issue with HT leads breaking down   or could it be something else…