I have noticed recently that my 1998 1.9tdi (professional conversion) seems a little down on power? I have a quality boost gauge fitted and normally (before the power loss) when the turbo goes to full boost it shows about 17 psi and then this backs off to about 15 psi. The gauge doesn't do this anymore and the turbo goes up to just under 15 psi but not beyond. All hoses seem good? Any thoughts?