Did you check the gas manifold pressure? Utility brings me gas at 6.5" WC and manifold was supposed to be 3.5" but mine was pushing 5" WC brand new. Who knows how it happened. It's a Honeywell valve on a Rheem.
Turn off every other appliances, or subtract their gas consumption on paper. At 75,000 BTU input, it should take 4 minutes to go through five cu.ft of gas. (assume 1,000 BTU/cu.ft). If its less than 4 minutes/5 cu.ft, you need to get the manifold pressure checked. It's over-firing if it's going through five cubes in considerably less than four minutes. If that's the case, the regulator on gas valve should be adjusted, then checked over the next few weeks(by clocking the meter) If it won't hold the setting, the gas valve needs to be replaced.
Oh and obviously, new filter and cleaning the blower fan doesn't hurt.