Hi..You say that the alternator has been checked ?...How did they do that ?...With a meter across the battery.
Is the battery running flat ? - Or is it fully charged and the starters just clicking ?..You might be looking in the wrong direction.
I assume that they put a meter on the battery then run the engine reving it up, if so the meter should show betwen 13 to 15 volts.
It sounds like an earth fault from the starter or a dodgy starter.
Recap your question mate, I will reply if I can