Yes, I had the same problem with the AT89C2051. In my project, it was necessary to generate one pulsed signal to the gate of a TRIAC, used as a switch for an AC non-resistive charge. So, every time the TRIAC was activated, the system was blocked. I tryed everything: RFI filters, reduced frequency, stable external clock oscilators, change the program and some external devices. In this case, I had to use a less eficient and more expensive solution: build a digital system without processors.
So, I thought that the problem was in use a processor for this kind of project. If there is really a problem with this device, it has to be solved.