I agree the first step is to replace #3 DV oring and crush washer.
Here's the 606.962 DV DIY. Personally, unlike what is shown in the DIY, I would NOT do this job until the IP was sparkly clean NOR would I remove anything from inside the valve EXCEPT for the spring and washer. Pulling the valve element and collar is a good way to end up having the pump re-calibrated on a bosch bench which is what your mech wanted to do.
http://www.peachparts.com/Wikka/OM606DeliveryValveSeals
If that doesnt help, I'd probably go back to stock configuration and troubleshoot it from there.