6n2 no top end

[SteveBall]

Hi guys, first post so be gentle as I have a Mini and I know nothing about my sisters Polo other than, it feels sluggish especially on a motorway, as in when you are going up a hill, this used to be my car and it never struggled before but now you have to drop it into 4th as 5th it really does not have any go. The cat had gone so it has been replaced along with the Lambda sensor and this has helped but it still feels awful. Is it possibly the temp sensor as my sister says somedays it is better than others.

Any help welcome.

Cheers