It is possible for a bad chip to cause stability or tuning problems,  however, it's far more likely that the external heating resistor is bad (or the transistor controlling it is),  perhaps a bad opamp in the control voltage circuitry,  or even just old electrolytic caps.   The ssm2033 is a really robust chip. 

I would make the assumption that it isn't the ssm2033,  and try to disprove your assumption by eliminating other possibilities.