agila when hot runs poor

I have recently bought a vauxhall agila 975cc petrol manual, had garage fit gear cables,oil filter, oil change,also they said someone before had fitted timeing chain but had damaged cam shaft so they fitted one cam shaft and lifters,since then the ecu light comes up code 300 and code 303...coil pack , ecu, plugs and injectors all replaced fault is still there when cold runs fine when warm cylinder 3 stops working and engine light comes on, compression check was good on all 3 cylinders when warm and cold.
spent out alot on this car now and it must be something really silly but what.
PLEASE HELP
many thanks
Trevor