Flickering of Monitors for Thin Client Computers

I am using thin client computers about 300 nos and there is flickering the in the monitors in most of the areas. All systems are connected to ups systems with isolation transformers. Transposing the thin client to a different area resolves the flickering. Transposing full computers PCs in the thin client area there is no flickering. Transposing cables from different areas there is no resolution. Overhead line of 33kV about 50m away.

What could be the issue and how does one resolve this

riyaz rangwala