well the prob. would have to be diagnosed, before it could be fixed. that depends on what the prob. EXACTLY is.
apart from the comparing thing i mentioned earlier, have you checked the dropout alignment? if its even slighty out, the one gear it work in well will be the smallest.
my shops to far away, (crowthorne), id do a webby search & trust your instinst's (instinks!?)
or..check my website, call me tomorrow (office hours) & ill help if i can
cristian.