Punchcard problem on Brother KH 840?

Hi, just dusted down my old machine after quite a long time in storage. Have cleaned and oiled it, replaced the sponge bar and general knitting is fine. However, when using punch cards, it sometimes doesn't select the correct needles or selects too many needles. Is there anything I can do? Does it just need a service? I am struggling to find anyone who provides this service now in the S Yorks/North Notts area, so any information welcome. Thanks!