Hey mustangpauli, it is definitely the Simple Trainer causing this issue as I had this happen to me. What you need to do is open up the trainer customisation file within the GTA V directory and within that file you can see which key triggers what action from the trainer. I simply referred to the mapping within that file (which 'code' refers to which key'), found the command which was triggered by the page up key and deleted the corresponding code so that it's blank and that solved it for me.