vauxhall zafira 2.0 TDI?

for 2 years my car will lose power with engine management light on it would go off after a few miles when i stop leave the car and return some times itwont start dosnt matter on time left then randomly it will go again and be good as gold for another few weeks then it will play up again i have had the diagnostic done three times no codes found, vauxhall tell me that if the management light comes on it will store a code, not my car one garage said the car dosnt like me another said it was because i use supermarket fule, im at end of tether as i have been standed by the car quite a few times