Mazda Miata: pop goes the radster

I have one of those once a week/2 weeks but never when at the garage
problems - such is life

Ocassionally when after a journey of around 10 miles I leave te car for an
hour, when i return to the car its difficult to start and when I give up -
return the key to its normal position I hear a POP.

If I don't have the problem starting I get a misfire,which disappears after
about a mile - any ideas ?

already had a new plug leads and the garage say they can find nothing
wrong - any ideas?

thanks all