First, have you checked the basics IE: non-clogged fuel filter (if excessive air bubbles get fed into the pump's inlet, it would - potentially intermittently - mess up the functioning of the timing advance system.)
How about your injectors - have you considered removing them, taking them to a diesel specialist and having him inspect how good their quality of spray is? Sometimes it seems bad injectors spray bad only intermittently.
Finaly, have you tried varying where you get your diesel from and/or trying some cetane-improver like stanadyne performance formula. Poor cetane and/or crappy fuel will clatter (what you might call "ping") more than a good quality fuel with high cetane level. Although it seems that this would lead to consistently high diesel clatter noise, not intermittent problem like you state to observe.
I've never heard of anyone actually diagnosing a bad injection pump timing advance piston, but I suppose it's possible if somehow exess dirt were to enter the pump, or perhaps if it were an assembly flaw - could have done it when new. If it were the case that dirt egress caused the timing advance piston to in it's bore however, I'd not trust the main plunger and it's bore - and would want to probably replace the pump with another good used, or new one. Even the prospect just to have a diesel specialist tear down, complete inspect the bores / other potential wear points, re-seal as needed, etc (essentially, that's what you call a "rebuild" is a several hundred dollar propostiion not to mention removing and reinstalling the pump. I'd recommend trying to diagnose it better, IE: rule out the easier stuff, like air bubbles in the fuel line first!
I also would take another look at the 1.9lTD crank bolt issue. I don't know much about it myself, but intermittent clatter isn't a common problem on 1.6l motors. It seems mighty coincidential that you've got a 1.9lTD experiencing this particular problem. Who knows maybe some cut corners when installing the upgraded bolt, or that did not fix the problem. Perhaps this is why the previous owner decided to sell their car. How about the updated/later 1.9lTD harmonic balancer pulley - do you have that along with the newer bolt or does the motor still have the earlier harmonic balancer? I'm just throwing out suggestions as to the 1.9lTD crank bolt issue, but I know from scouring ETKA parts database that there were 2 versions of harmonic balancers available for the 1.9lTDs.
Thanks Wingman, is there still people around 10:00 Pm cause that is the earliest i can get there. I work in Lachine an i finish at 21:30....What kind of car do you have so i can know who you are ?