v70 front oxygen sensor P0133

Hi all, I'm new on here and was hoping someone might be able to help. I've recently brought a 2002 v70 100,000 miles not long after having it the engine light came on after having it scanned it threw up a P0133 code. After having it reset a number of times I've noticed that it is only happening when i go on journeys of 20 -30 mins or more it isn't showing as a fault while driving only when I come to start the car again after parking.
I've changed the front oxygen sensor and maf sensor it doesn't sound as if the exhaust is blowing.
I've checked the 2 vaccum pipes coming from the air box they are clear and free from leaks.
Has any one any suggestions please, are there any more vaccum pipes anywhere?
any help appreciated.