how old is the house? is it a 2 way switched circuit? is it livened at the lights? it will be inductance in the circuit if its 2 way or livened at the lights been through this process with a client many years ago and i couldn't find anything wrong until i hooked a meter up and watched the voltage increase and then drop off as the cfl ignited, the only way around it was a to put a incandescent lamp in to stop it,
i found that with the inductance within the 2 way circuit it was enough voltage to slowly charge a set of filtering caps within the cfl lamp itself and then when the voltage reached about 50-60v it would discharge and give the flicker of the lamp