Chris,

BTDT> The starter motor heats very quickly-- 30 seconds is enough to
melt solder  which then is slung out of the one of commutator connectors
and gives you a "flat or dead spot" where the starter will not engage.
 If you hear the solnoid "clik" but no spin this may be your problem.
 BTW the extreme  rule of thumb is 30 seconds of cranking and then 30
minutes of cool down.  I know it seems kinda long, but ...

Sincerely,

Mark Keller
91 Carat