Is your refrigerator on Propane or electric? How much does your converter pull? It could also be the compressor start on one of your A/C units. Are they set to continuous fan where you might not notice the compressor start?
To really investigate, turn off all of the circuit breakers in you load center except for the A/C breakers. Let them run for a while and see if you still get the bogging. If not, turn the breakers on one at a time to find which circuit has the additional load that bogs the generator. Once you find the circuit, you can figure out what items are on that circuit to determine the culprit.