Volkswagen Passat starting probllems

I recently bought a passat. It was running smoothly. 10 days ago I tried to start car. Battery was completely flat. I got a jump start from another car. It started first time but car image on dashboard was flashing and the trip mileage read fail.after driving on and off for couple of hours it put itself right. 7 days ago again had to get jump start, not enough power. Went to shop about 10 mins went to car, tried to start it numerous times, it would rev for 2 secs and stop. Got towed home. Next day after charging it started straight away. It happened again yesterday rev for 2 secs and stop. In the end I tried starting the car in gear a few times and it finally started. The car image flashes on and off with the fail sign, please help, thank you