Hi I am not trying to frighten you but am merely pointing out what may cause the problems you have, the only realistic solution is to have the car scanned, the only problem with test driving is normally the tester drives like Miss Daisy so as to not alarm the vendor, when I bought my car at a dealers in Warwick we went for the drive with the dealer and my mate who took me up there I made sure the car was well up to operating temperature and drove round until I found a bit of dual carriageway and then gave it beans in every gear with my mate watching the back for smoke, the dealer was not impressed and asked my friend if I always did this to which he replied Oh yeah he is gonna be using it for a taxi and this has just proved it don't smoke, misfire ,or overheat and knock, all the ones I looked at got the same and if they did not like it I walked away. Steve.