Then why shouldn't I agree also.

I had exactly the same problem in an application using 2051.

It has to drive six Triacs to control six home appliances. Each Triac is driven by an opto 3021.

I used a limiting resistor of only 100 ohms at the LED of the opto.

It often locked for no apparent reason.

Then I changed the resistor value from 100 to 250 ohms.

The problem has been resolved.

It was over current sink into the micro that cause hang up and even the reset didn't work.

Chaudary Sahib, try this Chamatkar, hehehe

Good Luck

Babar/