Marc, if your receiver is new, then it is running on 2.33. Worth checking anyway.
The module stuff is a possibility as well. I've had a problem on my FG-12 module: it was not pushed enough inside the slot and was giving intermittent erratic orders and/or no transmission frames to the Tx module.
I use PPM-12 as well on my 12FG. The source frames are much higher than with PCM.