[earleygallery]

Just a thought, were you using flash when you had the missed frames? Is the battery fresh?

Also, hold the camera with lens removed and back open then fire, in front of a CRT TV or monitor. Try the same speed a dozen times in case its an irregular misfire.

Also, as suggested already, then run a roll of film though, while keeping the back open, to make sure it actually winds on and cocks the shutter each time.

If you bought from a camera shop they often provide 3 or 6 months g'tee on s/h equipment, worth checking.