My big turbo Esc Cos is really getting on my nerves here.

We installed some nice SPA gauges and things didn't exactly go like planned.
Now, after everything was done, we tried to get the car started wich of course --> FAILED.
Jump starting was no problem but it would not start on it's own battery which was new. We also had a misfire when boosting above approx. 1bar.
Changed several sets of plugs which of course didn't help at all.
We tried a new starter --> still the same.
In the end we changed the fuse box and she start now, although I ALWAYS have to try twice. She never fires from the first go.

We also thaught that the misfire would go away with changing the fusebox --> WRONG. The misfire is still there and now we just noticed that the fuel pressure DROPS to about 2.5bar when boosting.
Suspect is ofcourse the fuel pump but which pump should I get and can anybody recommend a place to buy and at what price?

I'd really like that my pride-n-joy drives again after hibernation from September 2003 onwards.