1999 Fiat Barchetta accelerator lag

i have recently had new valves and seals fitted to my barchetta, new spark plugs and cylinder seals and gaskets,the problem is that the idle is erratic, and the engine lags under acceleration,as though there is insufficient gas reaching the engine, i think the timing is ok, because the car starts 1st time, without any hesitation,could the problem be that the spark plug caps or the injection plugs have been replaced in the wrong order?