Kinda sounds like a venting issue. You should get a parts breakdown and an installation manual and see if the venting was installed correctly. I did one not to long ago, and the installer didnt install a baffle in the termination....or something lika that.
It made the pilot go out all the time.
“Now the freaks are on television, the freaks are in the movies. And it’s no longer the sideshow, it’s the whole show. The colorful circus and the clowns and the elephants, for all intents and purposes, are gone, and we’re dealing only with the freaks.” - Jonathan Winters