It ended up being the turbo (confirmed by my tuner/mechanic). This happened over 5 years ago. As a result, I replaced the turbo (with all supporting mods) and all exhaust parts except for the headers. Had my tuner put a relatively conservative tune on it and it has aged very well since then with no further engine problems.
At that time, it seemed like a lot of money but if I average that cost out over the past 5 or 6 years it really was not a bad investment.
70 to engine rebuild faster than you can say "poofff!"