Made a phone call today to a good mate who has worked with air guns all his life , he is resposible for designing the daystate air wolf , so knows his onions.
The problem is that they use a very heavy grease on the hammer (most pcp's use a dry hammer), over time it thickens and causes increased pressure at this point causing a varience in power. (there is a science behind this , but , dont ask me....lol)
The answer is to strip it , remove the grease , and replace with a light oil or light gun grease.
The reason for them doing this was unknown , but the later models seem not to have