The flight crew of an undisclosed aircraft type initiated a go around when erratic ILS signals resulted in a low altitude alert from the CLT Tower. A second approach was made with normal ILS signals.

Narrative

We were on approach to 18R CLT when after joining up on the auto pilot there were some erratic indications on the localizer. The plane would bank left and right 5 to 10 degrees due to sudden movements of the localizer. During the whole approach there was good identification of the ILS via the PFD. The First Officer decided to [follow?] the erratic indication and the sudden movements the autopilot would make.After glide slope intercept the First Officer reached task saturation due to this erratic information and then went full scale low on the glide slope causing a low altitude alert. Another approach was made with little to no erratic information from the same ILS. I am not sure what is causing this (aircraft or ground equipment?) but I have seen such erratic information displayed several times on 18R and 5 in CLT. It is not every flight and every approach as is evident by the second approach to the same runway and little to no problem with the indications. Such erratic indications can lead to mistrust of information and task saturation quickly.