This topic has been flogged pretty thoroughly before in these fora (do a search for "motor overheat"), but seconding Joe and Al's recommendation - choose gear ratios that will let your motors run at efficient speeds. Remember that motors convert electrical energy into mechanical energy and
heat.
To see how the drill motor speed is related to its efficiency, download the
Skill Bosch with Gearbox Performance PDF from FIRST.
There are a number of excellent white papers (in the technical section) on motor and gear ratio selection.