Help on cavalier pinking

Hi all,

Ive got a vaux cavalier 1.6ls 1995 and its has started to pink about 9 months ago.

Its really weird if I run the car with the "ron" slected to 95 and put 95 octain unleaded fuel in it pinks when the engine gets warm..Doesnt do it whaen its cold.

Now if I turn the "ron" to 91 and put 95 octain unleaded in its fine..but it used to run for years on 95 setting..

Ive just mot'ed the car and its passed..so as regards to emissions.and i put the "ron" to 95 for the test..

Ive done so far serviced the car and replaced the temp sensor on it to see if it was out to do with that.but nothing.And tried 97 octane i on the 95 octane and still pinks...:rolleyes:

Im thinking it could be either the knock sensor or lambda but im unsure which to try..

I really hope someone can throw some light my way,,the car has 76k on the clock for reference

Doe anyone also know if running the car on 91 setting and putting 95 octane fuel in will do it any damage

Thanks
craig