Intermitant power loss on 2001 VW T4?

Hi, It's a 2.5i 2001 VW T4. It has lost power though on occasions it kicks in. We have changed the airflow meter but this hasn't sorted the problem. I'm having problems locating the fuel injection solenoid. I was going to check for a loose connection, can anyone pinpoint where it is please. The turbo is workng ok. If not the solenoid what else could be causing the annoying problem. Thank you