Oxy Sensor for my VR Commodore

Hi guys,
i have just had to replace my 4th oxygen sensor in my '93 Holden VR Commodore, and i am wondering if anyone on here knows of any possible reasons they are burning out on me?

I have had several mechanics look at it, they even pulled the computer out to see if it had any faults but he couldnt find anything wrong with it.

So does anyone know what could be causing these problems?

Thanks in advance.

-Cheers

www.justcommodores.com.au

Ask them mate, they'll help you out heaps.