Generator problems

I have a new Nexus Super C with two AC units. Both work properly when on shore power but when the generator is running, either manually or in autogen, when the temp gets hot both ac units try to start at the same time, and it trips the relay switch (?) under the bed and neither starts. It sounds like "thump, thump". It keeps trying to start with no success. If I shut off the back AC unit the front one will kick on and then I can manually start the second one. The problem is overnight when they both shut off they won't come back on until I shut one down and manually kick it on after the other starts. It kind of ruins any hope of a good night's sleep (argh!!). I had the generator into Oman and they say it is fine. . . any ideas?