Burnt exhaust valve...again

This has probably been addressed here before but...
I have a '92 Metro 1.0L 3 cyl that twice now has burnt the # 3 exhaust valve. The first time occurred at approximately 130K. The motor has been replaced and 80K later the valve is burnt.

I have heard alot of reasons this keeps occurring: lugging the motor, poor fuel grade, fuel mixture too lean, retarded ignition timing, plugged EGR valve, exhaust system back pressure, too thick oil grade, etc. etc.

Could anyone please provide any insight as to why this keeps occurring?
Thanks,

high cylinder temps are what burns out valves. When an EGR isn't functioning properly, the cylinder temps rise, and take their rage out on the exhaust valve when it opens. Check your EGR, and all the EGR passages.