eea need help

hi every body,my wife is eea national exercing treaty right as worker but the probleme she is woking for agency,some weeks work and some not (3weeks she has pay slips the 4 not)and every 3 mounths she go back to her country for one week or two
she is due to apply for PR next year,we scare that the HO ask about this period when she wasn't working.
what we can do for cover this gap or at least cover the future gaps,shall we pay the fumous comprehencive sickness inssurance.

thanks for you time.