Electro magnetic clock

Hi

I have had the clock running for over 6 months now but have struggled to get it to keep to good time , it regularly gaining between 5-7 minutes a day , but then running to time for a week or so

I was sitting quietly watching the clock and it suddenly pushed the second hand ahead 2 seconds ahead for several ticks before going back to normal

it has happened twice in 15 mins which accounts for the time gain , but not sure how to cure it ,

the static and active pawls are set as best I can with only about 1mm backlash . but this would not account for the sudden double ticks

ghe Second hand is not always in the same position when it happens as I thought it may be unbalanced and pulling it round

any ideas ??