New cps is probably in order, preemptive.
And, clean the two important ground wires located on the passenger cam cover. This is a simple step; its where the engine electronics all ground as you should remember on S2. CPS and grounds / CE cleaning solved missing for our S2.
Another possibility is an intermittent contact fray at an injector or a fray at MAF plug.
Possible dirty ground, fuel pump, behind right rear quarter.
I find every 2 years needing to pull the CE panel and polish the fuse holders, and re-fuse. Relay replacement for the critical ones might be in order, too. LH, EZ and Fuel pump relays.
http://forums.pelicanparts.com/porsche-928-technical-forum/586148-ground-cleaning-16v-euro.html
Separate issue, Cutomer car --- have the fuel lines been replaced?