I think it was the intake flaps causing my troubles. Last weekend I took off the crossover pipe and verified that all flaps are working correctly (including the EGR flap). I even unplugged and re-plugged their electrical connections. Now the car (knock on wood) runs noticeably better even though I didn't really change anything.
Power is up again and the pinging is about 95% reduced (only one small ping today on the way to work). I haven't noticed any more surging either which would sometimes accompany the pinging. Maybe I had a stuck flap before and my testing unstuck it? They were all lubed when I did the nozzles.
gsxr, I have run DP through my engine many times, last time just a few months ago before I changed the nozzles. I also use an injector cleaner and cetane booster at every fillup. When I said I "eyeballed" my timing, I did it by checking whether the V ridge in the IP's side opening is centered at 15 degrees ATDC and it was. I know that's not totally accurate but I think it's good enough for just an inspection without adjustment. I agree ideally I should pop test my injectors now that they're broken in, but I highly doubt they're out of balance (based on my past pop testing & current engine behavior) and I don't feel like taking them out again which is less than pleasant on the OM606.
