I for one would like to know what exactly is failing? I assume the compressors. There are a few reasons compressors fail, poor installation and charging mistakes are one big reason. Has the superheat at the compressors been checked, if so what is it? What about the sub-cooling? Delta T? Low voltage is never a good thing for ac compressors. Maybe a buck/boost transformer would be warranted here or you may try and see if some hard start capacitor kits would help. The hard start cap will help a compressor start that is either suffering from excessive vd or starting to fail. Obviously, keeping the condensor coil and evap. coil clean is necessary as well as other basics. I doubt the hertz issue is causing your problem, undervoltage much more likely as well as poor installation practices.
