boost problem clarification

[Dan Simoes]

Took a run with Brett at lunchtime, I think this will help clarify:

1) Sometimes full boost is not possible, ie only 1.3bar, esp in lower
gears.  Other times it is normal/high.

2) When full boost is attained, it is often hard to hold it.  We saw
boost stumble after being at 11-12psi for a couple of seconds and cut
back hard to 7psi.

Clearly something is affecting boost.  My question - what could be
telling the computer to cut back or restrict boost?  I know all the
obvious answers, I just hate having to blindly replace every part.

Could a knock sensor go bad and not throw a code?

I will still try swapping WGFV and ECUs perhaps.