1.4 TDI engine start problems

Hello together, I hope to find help on the way. The Seat Forum is not so active. It is about a Seat Arosa 1.4 TDI Bj 2003 with 230,000 KM on the clock. At the vehicle was retrofitted by the previous owner DPF, so has the green plaque.We bought the car with 100,000 KM. The car is used by my father, he drives every morning to work with it, one way 34 km. Car has always been driven warm and in warm condition it gets from time to time. Since about 10,000 a problem often arises: As soon as the car comes to 70 degrees or more and you turn off the engine, the engine can only be started after several attempts. It also happens from time to time, but it has started after a few seconds. If the temperature falls below 65-70 degrees the car starts without problems. Computer has been completed, but error memory is empty. Has any of you had the same problems? Thank you in advance Greet Maier