I have an ao smith pro max gcv 50 100, and I'm having similar issues. I googled it and may have found an answer. There is a ceramic screen called a flame arrestor under the burner. The air supply comed in through that. It has very smallmjoles and is suceptble to clogging with dust. I just cleaned mine be removing tje plastic screen at tne botton. And sticking in a vacuum cleaner crevice tool. I alsi taped a toothbrush to a long screwdriver and scrubbed off the underside as best I could, and vacuumed again. I'm sitting here watcjing the burner with my fingers crossed right now.
Question for you, how did you remove the access cover for the thermocouple? Mine pass throigh a tight orange grommet and it seems if i pull off the cover, it will dislodge the thermocouples inside, and i cant see where they should be through the little window.