2003 Vauxhall Zafira rough idle

It started back in December,it was serviced and running better than it ever had before, then the back box of the exhaust went, the engine light came on and it started spluttering when you went above 2000 revs, this only happened very occasionally. Then in January the front pipe went, so i had that replaced, it then started spluttering and cutting out at junctions whenever you depressed the clutch the revs dropped to 5000, so i took it to the garage and had a diagnostics done and the coil pack replaced. Then 50 miles later problem started again. the car is fine to start and until the automatic choke cuts out, it then splutters and shakes when idling, when you press the accelerator sometimes it feels like there is no power, i spoke to the garage and they said the next step is to replace the ecu unit, does anyone have any other suggestions cause im not convinced.