Commuters are so frustrated they have even taken to social media calling the company "Chiltern Failways" on a Twitter site.
The problems started when a new service from Oxford Parkway was launched to Marylebone. They say a new timetable has meant fewer trains leading to massive overcrowding with many services late. On some days they say trains have been so crowded they have not been able to get on them.
Passengers say the company with some of the best punctuality - up to 97 per cent of trains on time in the past - is in chaos "and a shambles".
Stations with the worst problems include Gerrards Cross, Beaconsfield, Haddenham and Thame, Aylesbury, Bicester North, Banbury, Princes Risborough and Wendover.
Chiltern said it would like to "apologise for the recent dip in punctuality and is intoducing measures to bring it back to the company's high standard."
A series of measures has been brought in with more new times and the promise of extra carriages.
The company says the measures should help and it would continue to monitor the situation.