What can cause programmed NXT 2.0 motors to misfunction when small resistance is applied? I have robot built per the Alligator tutorial included with Mindstorms NXT 2.0. Everything works correctly if I hold it off the ground and make sure there is no load on the two leg motors. However, if I apply any resistance at all to the motors, then one of the motors will usually (~90% of the time) stop functioning correctly.  Whenever it stops it seems to be braking because it does not turn freely.
In the alligator program, I think it usually happens whenever the two motors are programmed to move simultaneously in the same direction for a certain amount of time.  If resistance is applied, then motor B will usually stop working. Of course, this makes the alligator basically worthless because it never ends up moving as there is always a little load when on the ground. I can typically get it to start working again by applying some manual rotation/resistance to either motor, but what works or not seems random.
I think I've noticed similar issues with other programs in the past, but I can't be sure. I have latest firmware v1.31 and we're using v2.0.f4 of Mindstorms Labview software.
My best guess is that the NXT firmware has issues with the motor control timing when two motors are moving together and one gets slightly off. However if that was the case, I'd expect everyone to have these problems (and the Mindstorms set would be basically useless.)
Are there any known/common issues that might cause a problem like this?  Any advice on how to troubleshoot?
It turns out that the issue is a faulty/weak motor connector socket.  While randomly trying to troubleshoot, I found that if I did nothing more than press or move the cable, the motor would act bizarrely.  Initially I thought it was a bad cable, but after trying additional cables it seems that it is more likely just a poor connection at the motor. I suppose it could also be a bad motor.  The actual cause is proving very hard to pin point.

And while one motor is definitely more problematic than the others, I noticed that all of them can be made to act randomly just by pulling on the cable ends.  I am now wondering if this is common issue with the NXT and I'm considering posting another question about it.

In answer to my own question, it seems that a faulty motor connection can cause erratic behavior.