Astra 1.8 keeps cutting out - any ideas?

My 2008 astra 1.8 auto with 25 k miles keeps cutting out, surges, feels lumpy but starts first time afterwards. It's a running problem rather than starting. Franchise dealer and own mechanic have donee code tests with nothing showing. Throttle body cleaned engine and petrol tank flushed with cleaners. Prob sill exists. Will be eternally grateful.

Forever in your debt
June 2013