*FACE PALM* It turned out to be the 10k resistor between COS and ground!
David's comment about the volume pin being miswired turned out to be the key to my discovery.? I saw that all of the cm108 ptt mods that include COS, have COS connected to pin48 on the cm108 chip (some with diode, some without). Pin48 happens to be defined as "volume down" on the cm108 datasheet. That clue led me to compare the COS circuit from several different mod schematics. I noticed most of them did not include that 10k resistor between cos and gnd. Unfortunately I followed one of the guides that did include the resistor. Not sure how/why this worked for the author of the article? I assume there may be some additional components on the COS line that aren't shown which make it work for the author (besides ground, which was also omitted from the guide).
Anyhow, after I removed that 10k resistor the volume dropping after the first tx is gone. I am 99.9% certain from all of this testing/research that the problem is now permanently fixed. Of course I will report back if not, but I think it's a safe time for me to kill this confusing thread. Thanks to all who gave me feedback. Also David, I think your comment about the vol pin is the key that led to the fix... so if you want that rpi ptt just let me know. It works 100% I swear!
