Ok, maybe someone with more Arduino experience can help me out with this, because it's making me crazy! I've attached two sketches. The one called MotorTest comes from the ADAFruit website and functions perfectly with by robot. It ramps up the speed of both motors going forward, then slows them down to a stop. Then it does it again, but this time in reverse. As I said, no problems with that. The sketch called navigation is the beginning of my navigation program for my new robot (see http://letsmakerobots.com/node/22697).