Studio flash & *istDS

For several months I have been using my *istDS connected to Powerflash studio lights by a hot-shoe pc adaptor and sync lead with no problems at all... I recently purchased an Interfit EX150A flash head and have experienced the following . If I plug the pc lead into my flash meter and trigger the EX150A from there, it works every time. If I connect it to my Fuji S7000 again via the hotshoe pc adaptor and lead.. the EX150A fires every time. However when I connect it to my *istDS.. it fires first time.. then refuses to fire for the next 4 or 5 shots.. then fires... If I trigger the EX150A using the *istDS integral flash.. again 100% firing..It's baffling and frustrating.. Anyone any explanation or experience of the same problem.. I'd be grateful if you could pass it on
Thanx