It sounds like the font information isn't being embedded properly. I know a lot of the MT fonts will not embed (a licensing issue with MS). Go to File>Properties>Fonts. Does it list ArialMT and does it say embedded or embedded subset next to it?
ah ha, I just found the common denominator..
All the lines of text that won't display have a diameter ø symbol in them.
Apparently that doesn't want to cooperate?
Fantastic. I had the same problem Same error in Properties, in my case it was the degree symbol in a dimension on one layout.
I turned the dimension to text and exchanged the original Arial symbol for a Times New Roman one and it work.
I also made sure that all fonts were embedded out of AutoCAD 2011 (dwg to pdf.pc3 > Custom Properties > Capture some > Edit Font list > Check All)
Pdf works perfectly now.
The problem lies with AutoCAD, TrueFonts, special symbols, and the AutoCAD Native PDF driver 2008+. Open the print driver go to Custom Properties. Set Font Handling to Capture all.