If you can stand yet another post.... I just remembered a problem that I had 
that was very similar.  It was caused by the interactions between the 
modified sine-wave of the inverter and the switching power supply used by the 
equipment I was using.  I solved it finally with an isolation transformer 
between the two.  Most likely this simply acted as a choke, dampening the 
switching transients.  Two identical transformers connected back to back with 
enough VA would likely serve as well.  Good Luck, Mike.
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