I was told that I should probably get a new gas furnace because (I quote from service ticket): "Upon inspecting operations found flame becoming erratic when blower comes on. Used inspection mirror and searched as far as it could reach. I noticed significant rusting and what appeared to be pitting or small pin holes. Recommend replace furnace and/or entire system!" The furnace seems to be working fine. It is an older Trane (maybe 25 years),

Cracks or deep pits in the heat exchanger are a big sign to replace, this could release raw carbon monoxide into your how. The erratic flame by itself is a concern but not a huge one. It will get worse and is inefficient, can produce more dangerous fumes.

The erratic flame is a sign of a crack in the exchanger as well. Being that the furnace is 25+ years old it has definitely lasted its lifetime. I would strongly urge you to replace the furnace. The crack in the exchanger will open wider each time the furnace fires due to heat expansion and contraction.

I will agree, just the statement it is 25 years old, sort of stops me there. Newer models will run better and be a good cost savings.