This may or may not be an answer here, but it is at least worth investigating. As I understand it, some trunk systems are now using LSM modulation, which most scanners have a hard time decoding. However, a few folks have discovered that if you adjust VR4 while receiving the problem system, and watching the percentage decode, it improved the decode rate greatly. I suspect - I've never seen any documentation on this - that VR4 *may* be an alignment pot for the P25 decoder. This is logical - it's an older scanner, and it wouldn't surprise me in the least if, over time, it fell slightly out of alignment.
Here is a pic...
File:PRO96 Adjustments.JPG - The RadioReference Wiki
If you are hearing this digital noise, and the CC is still relatively strong, it's worth trying....HTH...Mike