At some level we are all grasping at straws here. You are unfortunately in a situation where you need to make do with equipment that is being applied outside of its specification range.
It may simply be luck that some units work and others don't.
Perhaps the difference between the units that work well and the units that fail is the cycle time on the thermostat.
When an air conditioner first starts up, all of the Freon is at the same pressure throughout the system. The compressor starts and builds up the pressure difference that makes the system work. Once the compressor stops, the pressure bleeds back down to equilibrium.
If you try to _restart_ the compressor while there is still a pressure differential, the motor has to start against a significant load.
Combine this with the low voltage and excessive frequency, and the compressor motor might simply stall.
Perhaps some of the thermostats are cycling off and on too frequently.
-Jon
