PID controllers

What do you mean by "hanging"? I work with PID controllers everyday. They are so inexpensive these days, that if one stops working, I replace it with a new unit.

I personally like the Honeywell UDC series.

http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/controllers/udc3200/default.htm

At system level: if this behaviour is typical of the model, then I would change to a different model.

At design level (i.e., internal to the unit): a system component called a watchdog counter is used to monitor microcontroller/microprocessor activity. The watchdog kick-starts the micro when it hangs. Some microcontrollers have onboard watchdog counters, while others will require an external monitoring circuit. From a design perspective, first prize is to tackle the cause of the latch-up.

It can be caused by Harmonics in your line. Too much use of electronic chokes or having thyristors controlled chargers in the supply line, can result into harmonics. Please get it checked if you have these .