WOW ! sounds like a fun one to track down....

DO u have a MultiMeter, and can you measure the 240Vac at the Mains cable that is powering your flash?

Then, turn ON all your other machines, and note if the 240 is dropping down to 200V or less, if it's dropping down then you have a dodgy wire somewhere and losing current probably due to lack of strands in a badly wired plug, or shonky 4-way outlet board....

I'd like to hear more about how many extension cords you're using, and how many 4-way breakout boards etc etc...

This reaks of dodgy-ness