Well his 602 looks a lot like my 603 with one cylinder chopped off.
So I will go out on a limb here:
My 603 will rock the car everyonce in awhile. Usually if I bring the rpm's up the problem will go away. I just live with it for now. I talked to a Bosch shop about it and they said something in the IP could be getting weak and to turn up the idle. I turned the idle up and it worked now it only does it about half as much.
Afaik the only fix is an IP rebuild and I am not going to do that at this time.