_some_ Underscores not printing

[graham_harris]

I have a number of printouts where Textpad has omitted some, but not all, underscores between parts of identifiers. This is not good!

I guess it's something to do with variable line spacing- the lines with missing underscores have also lost the bottoms of tails of characters like p, j & q which drop below the baseline.

I'm using Courier font (I need a monospaced font because it's program code) and switching to Courier New at the same size makes no improvement.
The printer is a Fuji Document Centre C250 PCL 6.

Any advice on how I get my underscores back?