Why do I have a reduced flame in my natural gas fireplace when the furnace or water heater runs?
It sure sounds to me like a problem with the gas pressure or regulator/meter. Since gas can be dangerous if leaking into your house if a pilot blows out etc, I'd be on the phone to the gas supplier. They should be able to check the input pressure and assure that the regulator is working properly. Trying to trouble shoot and repair this type of problem is not a job for a DIYer. Here in Maine, only a licensed pro is allowed to work on gas lines. I am a general contractor and if there is a problem, I'm on the line to a pro immediately.  Good luck.