Ah yes that rings a bell now, faulty earthing which caused signal failures for a couple of months
It wasn't faulty. It was exactly to design - and was a temporary stage. However there was some 'noisy' background earth returns in the area (non-railway) which were not known about that caused issues with the temporary arrangement. This was the root cause, but took a while to find as what appeared to be happening was slight variations in the power supply frequency. It took about 2-3 weeks to discover and was then the final design was immediately implemented (earlier than planned).