"My bigest question is why one series of the chip work and the other doesn't.
Is it possible that we got a defected chips???"

Much more likely is a flaw in your software design; eg, you have relied upon something which just seemed to be consistent in the chips you tried, but was never actaully guaranteed in any specification or datasheet.
Uninitialised variables are a prime example of this - especially uninitialised Pointers in 'C'!!!

"Has any one experience with this before???"

For an excellent example, see this Thread:
http://www.keil.com/discuss/docs/thread1617.htm