Meandering Pitch, Where’s The Problem?

As I’ve mentioned in other posts, I’ve been modifying a Thorens TD 160 MK1. It is and will remain a dedicated mono table. Currently, I’m using the original TP 16 MK1 tonearm with an AT 33 mono cartridge. I will eventually replace the TP 16 with an Origin Live tonearm and EMT mono cartridge. I still however like playing records while in it’s current state. What I’ve experienced with a number of classical records is pitch instability. Primarily when playing piano recordings. There are certain passages that are a problem and I’m not exactly sure the cause. My cartridge is aligned using Baerwald. I understand that with some records, especially very old mono vinyl, that the pressings could be off but if that were the case, then the entire album would be an issue. I do need a new belt but I’ve still had this issue with a brand new Thorens or Origin Live belt. Just wondering what others think and if there’s an antidote? Thanks!