This is most often caused by loose or corroded battery treminal
connections. If these are ok, it is usually the ignition wire going
from the ignition harness(behind the ignition switch) to the start
terminal on the starter(the little wire, not the big one coming from
the battery). To check this, disconnect the small wire from the
starter, connect a test light or mulit-meter between the small wire and
the black(-) terminal of the battery. Have someone turn the ignition
switch to the start position and note if the test light lites up. If it
does, the wire and ignition switch are ok, if not the wire is bad.