Should you explain the reason for a lot of 'job hopping' in a cover letter or leave it off applications since the jobs are not relevant?

I'm not a human resources manager or anything like it, but here's my two cents worth. I have been told by a college instructor and others that do not offer too much information in a cover letter, which would be the most likely place for explaining things. Only write about your strengths, if they want to know about your hopping around, they'll ask during the interview. If they don't want to talk to you because of the hopping, then I would doubt your explaining will change their minds. Hope this will do until we can get a more qualified answer. If the application form uses a "chronological " method of listing your employment, any holes in your information will stand out badly. As a interviewer, I look for that every time I do an interview, and ask directly "why are there so many un-explained holes in your employment record'? To me it indicates the applicant is either hiding something, or is just sloppy in their information.