running problems on saxo 1.1 v reg?

hi, can anyone help i own a saxo and as i accelerate through all gears on occasions the engine dips as if to stall i rev harder and it picks up again and i after drive at high revs, my thoughts are this could be something to do with the air and fuel mixture but i can restart the engine and it solves the problem but not always and it can run perfect at times. thanks