1999 Nissan Almera

Hi.
My 1999 1.5ltr Almera was having starting problems until it eventually wouldn't start anymore. I got it towed to my mechanic where I got a new starter motor put in. The problem now is that after this has been fixed the car will start perfectly but it is revving up and down. It revs to around 1500 and down again. My mechanic has now told m that this could either be a simple diagnostics problem whereby if I take it to the main dealer they can recalibrate the throttle position sensor. If that does not work he said it may be that the throttle needs changing. This was not a problem before I had the starter changed. Is it possible that the mechanic damaged this whilst working on the starter or may I have damaged it by not having the starter repaired before it completely cut out? Or is it just plain bad luck?
Thank you very much for your time.
Jack