What's is the exact model # ?
Are we talking about a 1232-10 or 123211D ?
It could be the voltage coming in also.
Is it properly ventilated ? it may be loaded with dust and needs a good cleaning, (on the inside) I would try a good UPS with true automatic voltage regulation.
You can still get parts for those old units but you would probably be better off replacing the control with a KX-TA 624.
The ups is a good investment regardles as you can always use it on a new control.
I would try a good cleaning and a UPS with AVR.