Hello Cyberteque,
My encoders are actually simple ones (not quadrature) and I am using them for two purpoes; to measure the distance travelled, and to make the robot travel in a straight line by synchronizing the drive servos. Since I presented the problem here I have found two physical causes for my problems and have corrected them.: (1) I sometimes powered the robot by USB alone and the voltage sagged to 3.7v. Now I make sure that the batteries are plugged in at all times. (2) One of the pull-up resistors was making intermittent connection in the breadboard. This allowed the encoders signal to float rather than going high. I repaired the circuit. Now all is going swimmingly until the next inexplicable hardware problem arises. : ) I am finding robot programming much more challenging than the usual programming in which you are dealing with well-behaved abstractions . Thank you for your input.