I had the same issue with mine when my wife ran her hair dyer or coffee maker but, it was only when we were above 5000 ft elev. I was told they surged due to the fact the factory settings are for sea level and we are never at sea level except if at the beach. I do not know it to be a fact but I thought I would try and take another campers advice, I had nothing to loose except a few bucks and some time. He told me to have one gen tuned at a shop that is use to tuning boats at high elev, 7000 plus ft and another at about 4000 ft elev.
I had a shop tune them up, even though they were new. I had one tuned for high elev by an out board boat mechanic in Big Bear because he was use to tuning engines for 8000 plus ft elev. Then the other for about 4000 ft. by another boat mechanic. I now use the choke adjustment to tune the gens to make them run smooth at what ever elev I am at.
Now, no more surging. Worked for me and have not had issues for seven years with the same gens. I do clean, maintain and run my gens for a min of 15min per month.
JMHO