If you do a search on here there is some info that might help. The screen in the high pressure pump needs to be dealt with. It is said it is not serviceable but there are some things to test before sending your pump in for a rebuild wnen just the filter is the issue. In my instance the filter was not clogged but it was affected by the ethanol in the fuel. What it did id soften the plastic strainer enough to allow the 50+ psi from the vst to expand the strainer in the bore causing it to restrict incoming fuel and lowering the high pressure pump output.

here is a good post.
HPDI Mechanical (high pressure) Pump