Had this problem with mine and it had me banging my head against a wall for days trying to work it out.
Firstly and i know it sounds obvious, make sure the back cap is done up tight. they have a habit of coming loose if you apply to much lube on the o-ring and it gets on the thread.
And secondly, turn the dwell up, if yours is still at the standard setting (15), turn it up between 3-5 levels(somewhere between 18-20) then give it a go. solved the problem on mine straight away.
Ash.