Those problems that suddenly disappear when you visit the mechanic and the non-turbo Proton Preve

The Proton Preve -  No Actual Photographs - Apologies, as I was too busy trying to help solve a problem with it

Earlier today I drove the Proton Preve 1.6 Executive CVT IAFM+(Automatic) - i.e the non-turbo version. The reason I did this wasn't because Proton loaned me a car to do so. I wish Proton did but they didn't. Anyway, the reason I drove this non-turbocharged or non CFE Preve was because my friend wanted me to diagnose a fault in his Preve. It supposedly would stutter and jerk after the car stops, enters into reverse, reverses then moves forward again. 

Now the thing was that after a good half and hour of trying to achieve the symptom mentioned by the Preve's owner I failed in doing so. I suppose this is something that some of us have experienced, where the car would suddenly suffer some failings but when it reaches the mechanic, nothing seems to be wrong. This has happened to me a few times personally and it is quite frustrating.