I have a few thoughts.
1) I would be tempted to try a Z reading between Live and neutral at the incomer of the consumer unit and outgoing connectors of the RCD to ensure there is not a high resistance fault somewhere in the neutral. It is a test that is not necessarily recorded as part of an EICR (Electrical Inspection). If the value is bad, then the PFC reported on the form will be the reading between Live and earth and a high neutral resistance may be missed. I am not sure a high neutral resistance would cause this but I know neutral problems can have odd effects and most things have been ruled out.
2) Is undervoltage a possibility? The 230 Volt regulators will prevent overvoltage and spikes, but will it do anything if the voltage drops to an unacceptable level? Less voltage means higher current, and more heat. I know many PC power supplies can cope with a wide range of voltages, but are the computers that have burnt out using power supplies that can run on 110Volts (American standard) or ones that require 230Volts? There are posts on PC web sites that suggest prolonged undervoltage can damage PC’s (mostly power supplies) and I understand that things like washing machines have a significantly reduced lifespan if the voltage is regularly lower than acceptable (one job I worked on had prolonged periods where one phase was at 195 Volts, one at 215 volts, and the third phase at 235 volts, so it can happen).
3) Is the supply listed on the electrical inspection described as TN-S, TN-C-S or TT?
TN-C-S with neutral supply problems may create problems.
) and also checking of any earth currents that may be present. Also worth checking the potential differences between the various computer parts and every part of the electrical system in your house also.
