You are at or below the recommended starting load for that combination, and N350 is not a very good .38 Special powder anyhow.
You were either getting incomplete combustion at low pressure and temperature or you were not getting as much powder as you thought you were. (Have you weighed the actual powder delivered or are you depending on the Lee chart for the measure?)
I understand your rationale for trying to do it all with one expensive powder, but as R. Buckminster Fuller said, "Simplify as much as possible, BUT NO MORE!"
You can check the powder charge more closely and increase the load a bit to save buying another powder but you are on the wrong side of the edge of suitability and safety now.