One more thought. I run mine with a grounded neutral and with the red plug on the outside receptacle so as to get proper polarity. Do you have a polarity checker you can use? I don't know if it will help but best to get everything right.
I run mine on full (eco off) when in parallel because that typically means I need a lot of current...the manual also suggests eco off when running high current applications.
I toiled over sending these back and getting a pair of Hondas but the Yamaha engine is so much better than the Honda that once I solved the AC problem with the capacitor I decided to keep them.